My own modest theory is that fashion magazines are to women what magazines about computers (and porno) are to guys -- they're fantasy books. It's just that women's fantasies -- many women's fantasies, anyway -- concern being photographed (ie., desired) and looking glamorous (ie., desirable). Where guys seem to enjoy imagining what they'd do to and with what's in the picture, women seem prone to imagine being what's pictured.
There's an additional fantasy element too, which is autonomy. Part of what women fashion-magazine fans seem to enjoy imagining is the fantasy of being found glamorous purely for its own sake. They seem to want to forget about the pleasing-guys element. There's a little defiance in the fantasy -- and you can see the defiance in many of the kicky poses and attitudes the models strike.
Perhaps something that helps explain the appeal of these images is that not only do many women enjoy imagining looking like these models, they enjoy imagining feeling like them too. I think guys often forget what a weighty and earthbound thing it can be, being a gal. There's so much dreariness to contend with: fatbags, hormones, moods, emotional agonies, etc. Women are weighed down by a lot of burdens, or at least they feel that they are, which is good enough for the purposes of my attempt at an explanation here.
The gals in the pages of fashion magazines and catalogs aren't weighed down by anything, not even flesh. They burst out of cabs, they leap onto sidewalks, they let loose with irrepressible guffaws, they're caught by insistent cameras looking their klutzy-but-charming best; they're tall and slim, and they're feelin' good and they're lookin' ready to dazzle. The girls in the pix get to enjoy the champagne-and-cocaine fun parts of being a grownup woman. They aren't saddled with fat asses and wobbly upper arms, with PMS, with no-good boyfriends and lecherous bosses, with imperfect features, with senseless mood swings, etc.
What the fashion mags are selling is, to some extent, a fantasy of play and freedom.
Which, come to think of it, is (in a general sense) pretty much what men's magazines sell too. Many guys enjoy indulging in fantasies about utopia -- a male utopia full of gadgets and sex-without-consequences. Many gals love indulging in fantasies about utopia too -- a female utopia, where the fantasizer is carefree and irresistably desirable 24/7.
My hunch: perhaps superslim-and-supertall are a visual representation of carefree-and-desirable.
Gymnastics currently seems to select for girls who, like fashion models, go through puberty late, but who, unlike models, stay short.