‘In 20 years of doing this, I’ve probably fitted about three 36B ladies who actually said they were a 36B,” says Kelly Dunmore, chief lingerie stylist at Rigby & Peller. It’s one of those ambient truths; we all know that what we think is our bra size is probably wrong. Josie Fellows, Kelly’s bra-fitting apprentice, walks me through this tactfully. “It’s often, 85% of the time, a big shock in the fitting room.” We are in the fitting room. “People think E is the biggest size there is.” That’s more or less what I thought. “They think of D as such a full size and they’ve been buying a B for 20 years.” Yup. “And to compensate for the fact that their cup’s too small, they go up in the back and think they’re a 36.” OK. “So you’re a 32D.” She fits me in a bra, and I have to hand it to her, they do look different, more like breasts, less like a sideways banana in a Waitrose bag. But this is absurd. I’m a 36B. Fellows says ruefully: “If you say to a client they’re a G cup, it’s like saying to someone who thinks they’re an 8 that they’re actually a 14.”

We will consider in due course what has led generations of women to think their breasts were a size they categorically weren’t. But this is also a story about a mistaken market. Bras have been misfitting for years; suppliers failing to approximate women’s actual shapes. Perhaps in the olden days, when materials were less yielding and there was a strong call for structure and upholstery, the lack of range was more explicable, the way you don’t expect to get a sofa in 90 different varieties (although … Loaf?). But more recently it has been hard to escape the sense of a generalised market failure, driven by fashion, which sees breasts as an impediment to design, not so much a secondary characteristic as a nuisance.

‘People think E is the biggest size there is’: Josie Fellows, apprentice bra fitter at Rigby & Peller. Photograph: Linda Nylind/The Guardian

As for the mass delusion among bra-wearers themselves, it’s partly cross-pollination: “All the glamour models say: ‘I’m a 34 DD.’ Well, they’re not, I can tell you – they’re probably an F,” says Dunmore. Plus, there are a lot of women simply changing shape and not keeping on top of it, because who would? But it’s no accident that we think of D and E as mega-sizes, F and G as outsize and H to K as non-existent, even though that is miles away from the truth. These F, G and H sizes have long been part of the high-end market. “They’re our bestselling sizes,” Dunmore says. “Marks & Sparks have just started to talk about F, which is what we’ve been doing for 75 years.” But on the high street, until 20 years ago, they were non-existent; even now, you can only get them in Bravissimo. Ellie Corney, a director of Bravissimo, says: “If you think about 1995, before the internet, it was such a negative experience for someone with big boobs having to find bras. There would be 200 really pretty bras in John Lewis and you’d say: ‘What’s available for me as an F cup?’ You’d be led to this dark corner and there’d be this horrific, flesh-coloured, thick-strapped granny bra.” I have a friend of one of these outlier (yet, in reality, incredibly common) proportions and, in the old days, when we were young and still bought stuff, it was like a group mission to find her a bra. I remember triumphantly finding one in a market in Uzès, in the south of France, only to discover it was actually €32, not a 32E.

Bravissimo is the entrepreneurial dream scenario, the gap in the market that turned out to be a chasm. Sarah Tremellen was 25 when she had a baby and set it up, according to the manager of her Oxford Circus store, Jess Jaddour, 28, “out of her own frustration. Ladies with average body size but big boobs just couldn’t find a bra.” (Note to prospective parents: the opposite can also result.) It started as a mail-order company in 1995 and opened shops in 1999, of which there are now 21. “It came from this really authentic place,” Jaddour says, “and right from the beginning we had this very clear objective, which was to give women the best choice possible, but also to have this celebratory tone, your body is absolutely great. Because women were being made to feel so bad about themselves. It was all: ‘Your size is really difficult, your body is the problem.’” The success of the mail-order operation was mostly on the demand side: it remained extremely difficult to get suppliers to produce what women wanted. “We’d say: ‘We’ve got all these customers, and they’d really like a plunge bra.’ And a lot of the suppliers said: ‘They’re not suitable. It’s really complicated. They won’t be supported.’”

A Bravissimo bra. Photograph: Linda Nylind/The Guardian

It is true that bra engineering, or brarchitecture, if you prefer, is complicated: “The bigger your cup size, the more work it’s doing,” Jaddour says. “It is a job. It’s not a T-shirt.” The couture brands have historically been better at big sizes, because there are more layers of complication; a bra that costs £80 will have twice as many components as one that is £40. “There’s a lot of engineering that goes into it, and the ones that do it well, they’ve been doing it well for centuries,” Dunmore says.

But there is also a patriarchal backstory (isn’t there always?) that stopped the market evolving to suit the people it was for. Corney ascribes Bravissimo’s success to the fact that “we’re women and we’re talking to other women. A lot of lingerie retailing in the past has been very much talking to men. It was ‘Hello Boys’ on the Wonderbra advert.” The consumer was conceived as a man buying underwear for a woman. And men do buy underwear for women, but you couldn’t subsist on it. It is a running Bravissimo joke that they have a rush on lacy red sets just before Christmas and they are all returned in January.

Bras do not have to be fantastically expensive in order to work (although you would struggle to get a well-structured bra for less than about £30). “I don’t agree that you should be buying your underwear where you’re buying your cheese,” Dunmore says waspishly. But you do have to specialise. Bravissimo, hitting brick walls with suppliers, started to design its own bras, and the logistical stock challenge is enormous: one bra, in one colour, running sizes 28 to 40 and D to L, as they do, would come in 90 SKUs (that’s a stock-keeping unit, or a single item for sale); if you do that in three colours, that’s 270. And if your USP is that you want to make every woman, whatever her size, able to find the bra she wants, that is an epic number of SKUs.

Bravissimo store manager Jess Jaddour: ‘We had this very clear objective to give women the best choice possible.’ Photograph: Linda Nylind/The Guardian

The modern bra-fitting happens by eye, not with a tape measure, and nobody uses the historical plus-four method, where you measure the under-bust and deduce back size by adding four inches (so if your under-bust is 28 inches, you would be a 32). This fit isn’t firm enough, and the backstrap, which should provide 80% of the support, can’t manage it, kicking the weight on to the straps and the cups, which, for anyone over a D, will give them sore shoulders, bad posture and back aches.

“There has definitely been an evolution,” Dunmore says. “We never in the UK or anywhere in Europe did a 30 back: the French thought we were mad when we asked for those. As a nation, we are smaller backed and fuller cupped than they are, and they also don’t fit as firm as we do. We are historically known as firm fitters in the back band. As soon as a 30 came on the market, they were selling out in days.” The cup should have no gaping but also no squishing, so if any part of the wire is sitting on your actual breast tissue, that’s too small; and if you’re spilling over the top, so your silhouette looks like four breasts, that is also too small. There is a residual superstition around the “big” letters, as Jaddour says: “Society tells us that G is massive, but a 28G would be a pretty small woman, and a 32H would look completely different on five different women.” Since properly fitting bras have come on the market, women have started to flag up other things that have never worked on boobs: fitted shirts, various dress styles, an array of pretty standard clothes that have never been designed to accommodate breasts. The working fashion assumption is, to put it bluntly, that women with a larger cup size are most probably fat, and this simply isn’t true. Most Bravissimo customers are size 8 to 18 – and the company sells a range of dresses, tops and sportswear alongside lingerie.

Women are changing, too – Dunmore says that younger girls are much fuller-breasted than they were 20 years ago; she sees 13- to 15-year-olds wearing a G-cup (“There’s no scientific evidence for this, but I’m sure it’s down to hormones, the prevalence of the pill”). Technology is changing faster. Materials have more movement, more breathability, more strength, and styles that may once have been impossible in bigger sizes – the bralet, which is, broadly, a bra that is also a top – now aren’t. But the real revolution has been the normalisation of sizes that were, in real life, already normal.