I worked for about two years as a "plus size" model, at a size 12. (If you don't know womens sizing, the average fashion model is a 0-2, and plus size in a store starts at around an 18.) Showing up for photo calls and being the only girl around who had curves was very, very hard at times.
A lot of the "skinny" trend came from the 90s. That's where heroin chic (see: Kate Moss in the early/mid 90s) started.
Most people have heard "The camera adds 10 pounds." Which is true, ish. The problem with photography is a person is 3-D. A picture is 2-D. So the camera sort of "smashes" you out and makes you look wider, simply because something has to give. Models and actresses want to look skinny in those pictures, so they make up for the extra 10 by losing 30. Or 50.
Designers (as in top end) don't like the models to "affect" the drape and flow of the clothing as they walk the runway. They want their models to be walking hangers. I never worked for any line like that (not that I could have even if I had wanted to.) For shows like that, they bring the clothes in and every item is a size 2 or smaller. If you want to work and you can't fit? You don't get paid. They motivate the industry quite well just by doing that.
For instance, I once went to a photo shoot for a "plus size" company and the largest item they had was an 8. That is a not plus size, even in fashion world. So, I left. I thought I would be upset, but I wasn't. That is how that world works, and the more expensive the clothing they're paying you to wear is, the worse it gets.
The reason I got out of it wasn't because it made me feel awful about myself. I didn't live in L.A, I wasn't working super high end and I definitely was not famous so work was sporadic at best.
Afterwards, I did come up with some interesting observations about body image. For every guy that likes a skinny girl, there is a guy that likes a fat girl. WOMEN are the ones that feel they need to be that thin. (Granted, there are jerks who tell their girl to lose weight, etc. but they are fairly rare.) Women perpetuate this without meaning to.
Men like what they like. I have known more than enough guys who have no idea why their girl is constantly dieting and worrying about her looks when he's told her a million times she looks fine.
I seem to have gone off on a tangent. My point was really just to point out WHY there are so many skinny women in media and how it happened.