Having had a lot of experience in the ad world (as magazine publisher, ad agency exec and currently on the client-side of media buying), I think it boils down to this:
I think most would agree that magazines/ads/media are fashion leaders – what you see shown as fashionable now will be on the street in a few months… Applies to men, women, pets, etc.
So why if we implicitly realize they lead us in clothing, shoes, jewelry and other fashion items would we even doubt they lead us in fashionable “body image” too?
If you look at perceptions of beauty in the 20th century and compare them to media presentations of beauty, you’ll see that perception FOLLOWS presentation… Magazines from the early 1920’s show flapper-esque bodies and fashion… Guess what became the trend? Magazines in the early 40’s showed more military inspired looks for women: broad shoulders, a little more “powerful” look, and generally a larger, curvier woman than the flapper era, etc… In the late 50’s they started showing the “Glamour” look and very voluptuous blondes – shortly thereafter, Marilyn…
Whether it’s a “conspiracy” I doubt. They’re probably just doing their job: fashion stylists and editors decide what the next “in” thing is because that’s what the audience wants to “know”… and then they present it. Most times it becomes the “in thing” shortly thereafter.
If you want to say it’s purely artificial, I’d agree with you. A small group of fashion doyennes has been allocated the power to determine what will be considered fashionable… But we can’t bitch too much: WE allocated it to them. If women (men, pets) started simply ignoring what the magazines touted, this whole round-robin would end quite quickly.