In New York last week I attended my first catwalk fashion show. I found the bubbling backstage hysteria, unfathomable lateness, social anxiety about who was sitting where, that grave frivolity when frocks meet finance, all pretty much as billed. But the girls! Truly, nothing prepares you for being within touching range of models performing that absurd dressage prance.

The deep valleys of their clavicles, knobbly shoulders jutting from a racing-back vest, the thighs no thicker than knees. I noted that the only one with an ounce of breast tissue was allowed to model the swimsuit; how skeletal bodies are mitigated by the round childlike faces. It was as if the lovely 12-year-olds I'd seen getting fitted for uniforms at my son's new school had all been stretched on a rack.

And this show's designer, I must emphasise, champions older and bigger-than-average models. “We like healthy girls,” I was told repeatedly. “And they're all at least 17.” No doubt. I even spied one piling a plate at the buffet. It was just me, a lumpen runway virgin, who felt they looked sickly thin. Fashion folk become immersed in an unworldly and extreme aesthetic; their notion of normal shifts. As one editor puts it wearily: “Now I don't even look at the girls, only the clothes.”

So it is a measure of how parlous things have become that Alexandra Shulman, the editor of British Vogue, has written to international fashion designers complaining that their sample garments - displayed on catwalks or sent to magazines for photo shoots - have in recent years gradually shrunk. Now they are so small, the only models who can fit into them are flat-chested, hipless, and so emaciated that Vogue is fearful of readers' horror if they put them on the cover. Even the superwaif Kate Moss struggles. Post-motherhood, post-30, she has newly acquired breasts.

Shulman's letter is brave: in taking on her own industry she risks scorn, snubs and precious advertising revenue. But she is also clever to cut to the heart of the long-raging size-0 debate. Before, the glossy magazines or model agencies were blamed or the models themselves made scapegoats. In 2006 the Spanish Government decreed that any girl with a body mass index of less than 18 should be banned from fashion shows. Many balked at the notion of gazelle-like teens being publicly weighed like livestock. Designers insisted that these were just naturally slender young women and that they sized their samples accordingly.

Shulman's letter exposes the dark truth, that the pressure to use überskinny models emanates from the designers themselves. In gradually shaving millimetres off sample sizes they force a model, who already has only a few ounces of body fat, to starve herself. To choose, perhaps, between Prada and her periods. (Italian and French designers are the most didactic body fascists.) And if a glossy magazine wishes to publish the latest collections by, for example, Dior these are the girls they must use. Indeed some newspapers - with less “edgy” values and older readers - now resort to retouching catwalk shots to make the models look bigger, less scary.

Of course, surviving on Marlboro Lights and half a rice cake is the accepted toll for playing in the world's finest dressing-up box. But the inevitable shrinking of the fashion silhouette aspired to by millions of young women poses greater concern. Recently, admiring old pictures of Ursula Andress cavorting with Sean Connery on the set of Dr No, I noted that emerging from that iconic white two-piece was a wedge of juicy brown tum. Contrast with Miley Cyrus, snapped recently in the Bahamas in a bikini - slimmer than Andress and only 16 - yet who endured a gazillion online snarks about her “jiggling thighs”. Women's possibilities have expanded infinitely since 1962, yet our acceptable body template has never been more defined.

But the question remains: why do designers want their creations displayed on such achingly thin girls? Perhaps in a bloated Western civilisation, where to be fat is most often to be poor, thin is the ultimate exclusivity, the marker of the elite. Thin is exotic, requires extreme dedication and self-denial. Thin is a victory of chic over base animal appetite. Now that luxury brands such as Chanel have democratised themselves with handbags and perfumes, superthin models reassert a justification of their price tags; their clothes are only for the few.

It is curious perhaps that while fashion is purportedly about sex, this ultra-thin image is repellent to men. Surveys always reveal that most men would rather sleep with a woman who was a stone overweight than a stone too thin. And a study this week - admittedly of Australians, not versed in haute couture - suggested that men prefer the average-shaped gal: a 36-28-38 size 14, far from the 32-23-24 stringbean average Vogue cover girl.

But it is women, not heterosexual men, who think that clothes simply look better on skinny broads. Once, when I edited a women's magazine, tired of letters saying “Why don't you put fashion on bigger women?”, I did just that. I did a shoot with a gorgeous, voluptuous size-16 model and showed it to a focus group of readers. “God, look at her huge bum!” came the cries. Women are hypocrites and self-loathers, bitchily policing each other more cruelly than any man.

And chief among them is Anna Wintour, editor of American Vogue. I saw her at the New York show, a wisp in lavender. Her fat-free figure means that she looks 35 in photographs, yet up close is rather frail. It is a shame that she is unlikely to sign up to Shulman's campaign, as only if Wintour - empress of the mighty US fashion business - demanded bigger samples would designers feel compelled to change.

But Wintour has always wondered why critics fixate on the anorexic minority, not the obese many. On a recent visit to Minnesota she met folks she described as “little houses”. Fat people to her are not even human, just blocks on the landscape.

Yet fat and thin are part of the same problem, a deep anxiety about our bodies and food: a screwed-up inner dialogue between binge and starve, guilt and indulgence, image and desire that fills way too much headspace in almost every modern woman.

Simply being average, a normal, healthy shape, neither dragged into anorexia by fashion edicts nor seduced into obesity by the food industry: that is the hardest size to achieve of all.

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