Odd things are happening in women's bras. In recent years, the average British bra size has jumped from 34B to 36D, which means that while women's backs have grown one size, breasts have jumped up two. Many department stores have increased the range of cup sizes on offer to meet the ballooning demand. In 2007 Marks & Spencer introduced the J cup. Earlier this year, Selfridges began stocking a K cup range, and its sales of D to G cups have risen by 50% year-on-year since 2005. Last week, Debenhams started stocking KK bras, which were previously only available in specialist stores.

In a country where one in three women is overweight, you'd think there was a simple, fat-related reason for this, but obesity alone doesn't explain the jump in cup size, nor the biggest growth area in bra sales: smaller back size and bigger cup size. Judging by recent underwear figures, there are more slimmer women with larger boobs than ever before. Women are happy about this. Men are happy about this. But no one seems happy to explain why this is happening.

Do you know how to work out a bra size? As roughly 50% of the British population wear them, you'd have thought most of them would have an idea. But though a 2009 survey found that the average British woman owns 16 bras at any one time and buys four every year, fitting them is a surprisingly tricky activity. The traditional method reads like an A-level algebra problem. You take a tape measure and wrap it round your chest at the lowest point where a bra sits. You record this figure in inches. You add four to this measurement if the number is even, five if it's odd – and the resultant number is your band size. Then you wrap the tape round again and measure the fullest part of the actual breasts. Next you subtract the band size from breast size to find your cup size. If the numbers are the same, you're an A cup. If there's an inch difference, you're a B; two and you need a C cup and so on. Alternatively, and many bra experts say more accurately, you can weigh your breasts by dunking them into a full bowl of water and measuring the displaced liquid, with 1 litre of water equalling 1kg. It's accurate but useless. You can do precisely nothing with this information, as no bra manufacturer measures boobs by the pound.

Unsurprisingly, as no one enjoys maths or physics homework, the modern way to find the correct fit is to go to a shop and get someone else to do it for you. Egged on by TV stylists, such as Gok Wan and Trinny and Susannah, who've long been rhapsodising over the merits of a well-fitted bra and the wonders they work on your shape and posture, more and more women are doing this. Previously they could go a lifetime buying new bras by guessing or simply choosing the size they'd always worn. They made do. But trained fitters can now be found in almost every lingerie department; instead of relying on water or tape they add an element of mystique to this already complicated process. Fitters are like boob whisperers, their pronouncements made on look and feel as well as measuring.

"We don't use tape measures because we don't believe they work. How can you measure volume with a straight line?" says Anna Prince, a bra expert for the Bravissimo brand, which specialises in DD+ bras. "Instead we prefer to show our customers how a well-fitting bra looks and what it feels like, so they will be able to tell in a matter of seconds whether they've got a good fit."

Marks & Spencer reports that about 8,000 women come to their stores for a fitting every week. As a result of all this bra buying and breast scrutiny, we have more information now about the dimensions of the average British boob than ever before. And what's surprising is how wrong most women were about their bra size.

"In the past few years we've seen a significant change," says Helen Spencer, lingerie and swimwear buyer at John Lewis. "The best-selling size in the business was 34B; that's what every woman thought she took. For us it's now 32D. That's happened in the past 18 months, and watching the sales coming in now, we can see that it won't be long before 32F becomes the biggest seller."

"There's been a huge growth in the small back, larger cup lady, particularly among young girls," says Julia Mercer, head of fit and technology at M&S's underwear department. "The younger girls just seem to have bigger breasts now."

The retailers' findings are borne out by Britain's breast biomechanics research unit at Portsmouth University. It's a niche academic field, but there is a team of researchers studying how much breasts of varying size move during activity and how trajectory changes with different types of exercise. Their aim is to improve the design of sports bras. "The media always contacts us when there's a new story," sighs Jenny White, one of this country's five breast biomechanics researchers. "And they always get excited when it's about size." But yes, they've found that women tend to wear the wrong bra size, particularly underestimating the cup. This finding cannot be explained by weight change alone. How do the women react to this news? "Well, there's still an idea that bigger is better, so generally they're very pleased."

The breast is made of glands, fat and connective tissue. Breasts make up about 1% of body weight, 4-5% of body fat. But the breast has always been more than the sum of its parts. That's why these new figures about bra sizes are interesting. If the average shoe size was up, no one would give two hoots. But breasts are sexy and thrilling – most of us want to have or to hold a perfect pair of boobs. Their preferred size and presentation are culturally significant, and the idea of what makes an ideal breast changes. Social psychologists have found that preferred size increased steadily from the flat-chested 1920s up until the early 1960s, when smaller breasts became more popular again. Research in the late 1990s found larger boobs were yet again more appealing.

We know that the way we regard them changes, but there hasn't previously been such a significant variation reported in breasts themselves. It's a peculiar moment of synergy, when culturally we like big breasts and women have suddenly discovered that they have them. Because big, bouncy, natural breasts are in – not the bee stings of sallow models that have made fashion in recent years so sexless, nor the overblown boob jobs that seemed, if you looked at magazines and films, the only viable alternative.

Fashion houses have ignored boobs for decades, but now underwear as outerwear is a top trend for summer, with every label from Dior and Bottega Veneta to Christopher Kane and Marc Jacobs pushing bra tops and corsets. And this spring's recent round of fashion shows featured many models who actually jiggled as they walked, rather than the size-zero coat hangers who usually rule the catwalks.

In Milan, Prada's collection celebrated curvier figures and accentuated the bust. In Paris, the Louis Vuitton models included recent mothers Adriana Lima and Karolina Kurkova, while models typically associated with lingerie companies also graced the show: Giles Deacon went for Victoria's Secret models Laetitia Casta and Bar Refaeli. Fashion hasn't wobbled so much since Vivienne Westwood's last platform shoe collection tumbled down the catwalk. Lara Stone has become one of the most talked-about models of the moment, not just because she's engaged to David Walliams but also because her gap-toothed pout and fuller figure make such a welcome change. Part of that appeal is her C-cup bosom, and she's not the only busty model who's finding favour. In the Forbes 2009 list of the world's top-earning models, five of the 10 highest earners take a C cup (Gisele Bündchen, Heidi Klum, Adriana Lima, Carolyn Murphy and Miranda Kerr, in case you're wondering).

Of course bigger doesn't always mean natural, but that does seem to be a requisite now. On TV, actress Christina Hendricks has become the standout star of the show Mad Men, partly because of the feistiness of her character, Joan Holloway, but also because of her mesmerising embonpoint. In film, Disney has also come out as an unlikely champion of the natural breast. A recent casting call for female extras for the fourth Pirates of the Caribbean film specified: "Must have real breasts. Do not submit if you have implants." Breast augmentations are also waning in popularity. They've been in decline in the United States since 2007 (though it still remains the most popular cosmetic surgery procedure in America), and the British Association of Aesthetic Plastic Surgeons (Baaps) reports that in 2009 the number of breast augmentations in the UK rose by only 1%, showing that demand for the procedure is slowing down.

"Ironically," says Dr Patrick Mallucci of the London Plastic Surgery Associates, "celebrities who have made their names on pumped-up breasts have become reference points for what people don't want. Clients often refer to Jordan and say, 'Don't make me like that.' Now people don't bring in photos of celebrities as a guide to how they want to look. They bring pictures from the internet, or of FHM models who aren't famous but who happen to have nice breasts. I just don't see patients who want massive knockers."

Instead of paying for them, women seem to have developed bigger breasts all on their own. Lingerie manufacturers deny that vanity sizing – when manufacturers alter the measurements of a dress size so that heavier customers can wear what appear to be smaller clothing sizes – is responsible for the seismic cup-size increase, but there has been something of a revolution in bra manufacturing in recent years. Bras are phenomenally complex garments and their construction has long kept designers and engineers busy. As an article entitled "Brassieres: an engineering miracle" from the February 1964 issue of Science and Mechanics journal states: "The challenge of enclosing and supporting a semi-solid mass of variable volume and shape, plus its adjacent mirror image, involves a design effort comparable to that of building a bridge or a cantilevered skyscraper." Though the experiments into the production of a plastic spray-on bra in a can also mentioned in the article sadly never resulted in viable underwear, this pursuit of new design and construction methods remain key today. Sue Dunmore is a designer for a new Australian brand, Dimity So, which specialises in fashion bras for the DD+ market. Her design degree was in 3D design, specialising in silversmithing. "What does this have to do with lingerie?" she says. "More than you would think, as they are both about creating three-dimensional structures. You have to be able to understand 3D forms to be able to create great-looking and fitting bras."

In the past, women couldn't wear the 32G or 34FF bras that are currently flying off lingerie department shelves, because they simply didn't exist. Katie Halford, founder of vintage-inspired lingerie label What Katie Did, which creates glamorous underwear from 1940s and 1950s patterns and materials, says, "Only 10 years ago it was nearly impossible to find anything bigger than a D cup on the high street, and things have certainly improved. In the 50s, cup sizes only came in B to D and if you were bigger than that you were encouraged to go up a back size instead – I've got vintage corselettes in bizarre sizes like 56B." Is it possible that boobs haven't actually changed that much, but that underwear has finally been developed to cater for figures that have always existed?

"There have been huge innovations in underwear," says Marks & Spencer's Julia Mercer. "In strap and bra technology and in control panels in shapewear, which allow zone control."

She explains that there are 31 components in a bra with a DD+ cup, 10 more than are required for smaller sizes. There's an extra panel of fabric in each cup to allow volume curve. There are slings down the side of the cups that hold the wearer's breasts securely to minimise movement. The underwiring bands have greater stretch to open up and frame the breasts. The underband and shoulder straps are modular, so that some parts stretch while others don't, to create an excellent fit.

"There's a lot more variety in trends and colours in larger cups now," says John Lewis's Helen Spencer. "Brands that are known for contemporary styles, such as Elle Macpherson's Intimates, have worked hard to produce good products for G-cup customers. There are styles with plunging necklines that achieve the same look as smaller cups – a lot of engineering has gone into them. The improved range of styles makes people more comfortable about purchasing these larger sizes. They're pretty bras."

But even if the underwear industry isn't pandering to vanity sizing, you can't underestimate the effect of women's vanity on their purchases. Modern bras may be feats of engineering, but all women know that if you want to buy a bigger cup size, there are easy ways to do so. "Bras are graded so that if you go down a band size, you go up a cup size. The cup size for the following band sizes is the same: 30F, 32E, 34DD, 36D, 38C," says Katie Halford. "In Marks and Spencer I have changed from a 36C to a 32DD in the space of five years, while I remain a 36C in designer brands."

Improved bra engineering has not improved the disparities in sizing between different brands. "It's a minefield of different sizes out there," says Portsmouth University's Jenny White. "There is no standardisation of sizes across the industry." Dr Patrick Mallucci agrees: "We hate talking about cup size because it's such an inaccurate measure. You'll be a B cup in one brand and a D in the next. It creates a lot of confusion when we're designing breast enlargements. We talk about form, proportion or shape."

Changes and sizing inconsistencies in the underwear industry aren't the whole story. There are various other theories posited in the media for this bosom expansion. Most of them seem to tie in with the most fashionable current cultural fears and are rather gloomy. Could bigger breasts be the result of binge drinking and bad diet?

"A calorie is a calorie no matter where it's from," says Sian Porter, consultant dietician and spokesperson for the British Dietetic Association. "True, there are seven calories per gram of alcohol and 50% of people who do count calories don't count those in drink, but in the end it all comes down to consumption."

There have also been a lot of alarming stories reported about xenoestrogens – manmade chemicals that are present in everything from weedkillers and food preservatives to make-up – which may be responsible for everything from increasing breast size and the early onset of puberty in girls to causing males to develop female sexual characteristics. They may yet be revealed to have wreaked havoc on the biology of the human race in the imminent future, but right now there's apparently not enough conclusive proof to say either way.

Dr Ian Fentiman is professor of surgical oncology at Guy's, King's and St Thomas' School of Medicine in London. His main area of research is breast cancer, so he's studied the effects of xenoestrogens because, yes, they've been implicated in the development of that disease, too. "There's little direct evidence but lots of indirect evidence that xenoestrogens have an oestrogenic effect and affect the growth of human breast cancer cells; but to take that on and say what the impact of this is on human females, well, it's a wee bit more difficult to say," says Dr Fentiman. "We're doing some work which seems to show that you can see evidence in relatively young women that they've had exposure to a variety of mutagens and the body has had a response to those. It may be that if you get an exposure to xenoestrogens at a young age there may be an even more profound effect. The young breast is very sensitive to a whole variety of stimuli. But none of it is yet proven."

Sian Porter says that breast sizes are definitely on the up, but that's because of a very simple reason. "We are all much better nourished," she says. "Each generation is getting bigger, taller and stronger because we've eradicated a lot of childhood diseases. So our mothers' generation are bigger than our grandmothers and so on. It's a long-term trend. If you have the capacity to grow to 6ft yet you're malnourished, you'll only reach 5ft8in. If you're well nourished during the period when your secondary sexual characteristics develop, you'll reach your potential. The babyboomers were the first NHS babies and from then on we've been in better health – apart from the current problem with obesity."

And that's the unavoidable bad news. "Bra size will be affected by your skin and muscle tone, a variety of factors," Porter adds. "But the message has to be that if your clothes are getting tighter or you need bigger ones, you should think about losing weight." Sub-scapular fat – or back fat as it's better known, the rolls beneath your shoulder blades – can be an indicator of weight problems. It's one area, along with hips, triceps and biceps, that doctors will measure with skin callipers if they're measuring body fat.

Dr Fentiman concurs with Porter's theory. "We know that breast size is changing, but so is the age that women start menstruating. In 1900, the average age for menstruation was 16; by the end of the century it was coming down to 11-12. That's a reflection of reaching a certain body mass and that's because most people in the western world have easy access to food. It's not breast size alone but also body size that is increasing, so nutrition is currently the most obvious explanation. Yes, there may be other explanations as well. As with all of these things, science lags a long way behind. We can make observations, but to explain why those things are happening is more difficult."

At least, for now, it seems the majority of women are pleased to discover their hidden assets. Julia Mercer of Marks & Spencer says that one thing she's noticed among the women coming in to buy the new bra sizes is that they're proud of their shapes. "Minimiser bras used to be popular," she says. "If you had large breasts you used to try and hide them – now more women are celebrating their breasts." Helen Spencer from John Lewis reports a similar experience with her customers. "In store, I see a customer who's found the right size and they're surprised by how fantastic they look. A few years ago, what was out there wasn't very exciting, but I honestly think that now everybody's just making the best of what they've got rather than settling." Sue Dunmore at Dimity So concurs: "Consumers these days are used to having choice in every part of their life; just because you are fuller busted doesn't mean that you can't choose to wear beautiful, sexy lingerie just like everyone else."

The big boob bonanza of 2010 may simply be part of the ongoing evolution of the female form, but it's one thing women are content not to have to worry about – at least for now. If you've got them, you might as well flaunt them.