Where there’s a problem, there’s a profit. And tonight, a rather horrifying Channel 4 documentary called Plus Sized Wars illustrates this, going behind the scenes as the High Street fashion industry battles for the prize of a special teenage market: the seriously obese.

We all know girls are getting bigger. It started gradually, as waists grew and female musculature changed with better health, exercise and nutrition.

As average sizes grew and tiny, cinched waists became rarer, we saw a number of years when the chain stores stealthily extended their sizings. Come on, girls, admit it: in some crafty shops, ‘size 12s’ are what we once knew as 14/16.

Models present creations during the Plus Size Fashion Weekend 2015 Summer Collection in Sao Paulo, Brazil last year

But this is wholly different. Now, a group of labels are getting out-and-proud about bigness, targeting designs at the ‘fat and fabulous’ and recruiting poster girls of startling dimensions to advertise the clothes. Demand, they say, is ‘going through the roof’.

However, businesses are not doing this out of kindness. The harsh truth is that Britain is the second fattest country in Europe (Hungary wins) and it is estimated that 60 per cent of teenage girls are overweight.

That claim can be taken with a pinch of salt — the Body Mass Index scale on which it’s based has lately come under criticism.

BMI, which has long been the measure of what is normal, is a simplistic tool that simply plots your weight on a graph alongside your height to determine if you’re overweight or not.

It ignores how much of your weight is muscle, thus putting very fit athletes into the ‘overweight’ class, and apparently, for mathematical reasons, tends to overestimate the BMI of taller people.

So FINE: accept that some of the hysterically claimed 60 per cent will just be sturdy, active, perfectly normal, but rather unfashionable size 14/16 girls.

Yes, they may struggle in clothes shops because male designers are over-keen on superskinny models and the boyish look, but the Channel 4 documentary isn’t talking about them.

They’re beyond that, by far. Indeed, one designer says flatly that these newly targeted customers are girls who ‘can’t relate to a size 16’.

The poster girls for the new designs are — despite their tumbling, lustrous hair, peachy skin and confident swagger — frankly enormous.

Plus size models pose on the catwalk at Pulp Fashion Week in Paris last year

Not plump and playful, but seriously huge: carrying the kind of slow-moving, 25st-plus obesity that alarms doctors and stores up endless health problems for their future.

Through these young role models, the industry is selling to the young the idea that a super-fat body is not a problem. And that is something to worry about.

In the programme there’s one of those endearing old Pathe News bulletins about a fashion show for older women with thickening, matronly waists.

The commentator jokily chirps: ‘So ladies, eat what you like, and lots of it!’ That was once the norm: svelte girls blew up a bit as older housewives.

I’m not saying that middle-aged spread is a good thing or a problem to be ignored (even if we have just been told by scientists that a bit of middle helps stave off Alzheimer’s).

But we who carry a bit more than we should in our 50s have at least had a turn at life.

We’re grown-ups. We know that with a bit of time on the 5:2 diet and scampering around it is possible to get a bit narrower, and that we ought to.

It is far more ominous to see increasing numbers of teenagers punishing the scales and needing clothes from a special rail.

Not the size 16 rail, but the 22 to 30 tentwear. It’s likely to make them miserable, whatever they say and however glorious the role models. It’s a danger to health. And it’s not comfortable.

The problem with feeling unhappy, unfashionable and out of the swing of things is what the manufacturers and designers are working to alleviate. Or, if you like, exploit.

One of them is Evans, formerly known with brutal frankness as Evans Outsize and possibly the most depressing chain on the planet, full of sweaty acrylics and boxy jackets clearly designed for Spongebob Squarepants to wear at funerals.

Now it’s smartening up, heading for the bigger youth market, which buys clothes more keenly and more often.

So model agencies are looking out for large but gorgeous girls, scouting for them on the increasing number of plus size blogs, where they post pictures of themselves in favourite outfits, smiling or pouting. Some plus-sized-models are getting worried. One agent remarks that they come to her saying: ‘Oh God, am I not working because I’m not big enough?’ They even pad themselves out.

The models who were interviewed in the TV programme are, for the most part, relentlessly positive. Hannah Boal, a blogger with more than 4,000 followers, says: ‘I used to really hate myself.’ But in a series of huge, bonkers wigs and colourful outfits, she now has swagger and confidence.

‘I’m here, I’m who I am, I can wear what I want.’

Another says: ‘I will never be the right weight unless I get ill. Last night I had my cheese feast: I ate three kinds of cheese. And I don’t deserve less respect because of that.’

You’d need a heart of stone not to be glad about their positivity.

Why, asks a fashion industry producing bright, exciting clothes for large silhouettes, should these teenagers be condemned to avoid white and stripes, and stick to navy and black dresses cut like circus tents?

They’re young, they need to go out and feel good and make a splash. And certainly it is true that if they are given courage and confidence to show themselves, there will be more incentive to move around a bit, dance, get active, rediscover what the body is actually for.

But then there are some quiet, thoughtful voices among the pretty young bloggers.

Georgina, filmed in a gym with her trainer, is a sensible girl who just wants to get down from a size 18 to a 14/16 because she is getting married, wants to have children and be in decent shape to run around after them.

‘I don’t want to be that mum catching her breath because she’s not physically fit,’ she says. ‘I am not going to be fat and proud.’ But the response from her followers is not entirely sympathetic.

Some say they ‘feel betrayed’ as if she is abandoning a tribe, a minority, a serious cult.

Any schoolgirl will recognise the artful desire to keep friends as big as you are. But how must it feel to lose followers and attract spiteful comments just because you want a normal, active family life?

And, of course, when the splendid Georgina does get down to the healthy, curvy size she wants, she will be despised by those followers and bombarded from the other direction by the contempt of the mini-Gwynnies, the anorexically skinny role models.

She will feel shamed all over again for not losing more, and read stupid interviews where some ditzy celebrity says in a shocked tone that before she discovered a faddish diet of raw turnips and green slime: ‘It was terrible — I had ballooned to a size 12.’

So there we have it. The ridiculous obsession with women’s dimensions can, emotionally and literally, bend us out of shape.

What is never sufficiently said is that there is a healthy, cheerful, sexy range of female sizes between the ultra-fashionable (and often anorexically infertile) skinny lizzies parading their eerie ‘thigh gaps’ on social media and the dangerously enormous group targeted by these fashion chains.

Neither extreme is at all helpful to women. The over-praising of the stupidly thin has been damaging, and this growing glorification of the ‘fat and fabulous’ risks doing it in the other direction.

On the Channel 4 programme airing tonight, one model agency boss looks at her ultra-plus girls and says defensively: ‘I’m not a doctor, that’s not my job’

It could discourage teenagers from getting off their backsides and putting in the admittedly boring effort to eat properly, take exercise and fight off whichever minor addiction — booze, cupcakes, pies — is causing the trouble.

And it really is trouble when a person of either sex soars above a reasonable weight. We know about the health risks: diabetes, heart problems, strokes, breathing difficulties, even some cancers.

On the programme, one model agency boss looks at her ultra-plus girls and says defensively: ‘I’m not a doctor, that’s not my job.’

But what also needs saying about obesity is that long before the final price is paid in later years, there are daily discomforts and miseries. Behind each of these beautifully made-up, artistically shot pictures of these larger girls in colourful clothes lies inevitable, unacknowledged discomfort.

There is one depressing sequence when a designer boasts of providing special long knickers for those whose thighs can’t pass one another without painful chafing.

But this isn’t a fashion issue: when walking becomes unpleasant, you have a vicious circle that probably ends up on an otherwise unnecessary mobility scooter, rolling along the pavement to the shops.

Anyone who’s been too big and then lost a couple of stones will tell you how revolutionary it is in terms of comfort and mobility.

I’ve been a variety of sizes — though I never got near the size of those models the programme glamorises, because I like springing around on boats too much.

Even so, I can confirm that the bigger you get, the sweatier and more tired you are, too.

And how can the biggest relax on trains, planes and buses or in cinema and theatre seats?

How can they feel OK when the person who has to sit next to them is clearly resentful or embarrassed by the hot, awkward overspill? For many, even a few months of this is enough and they take action.

No matter how much glorious hair, peachy skin and funky clothes you have, nothing is enough to make life comfortable at size 24.

If beautiful, sassy clothes get big teenagers feeling that they matter in the world, that’s great. But you have to hope this confidence will extend to also accepting they have the power to lose a few stones, like Georgina, and bounce through life rather than having to waddle.

Good luck to them all.