I think fashion models tend to be overly skinny because the fashion designers don't want the women to be overly-sexed.  A really sexy woman with curves would take away from the focus being on the clothes they are trying to show off--people would be too busy looking at their breasts. 

I'm actually more irritated with Playboy.  They've really promoted the plastic surgery industry in the past decade or so.  Women who don't fall into the C-D range feel like there is something terribly wrong with their breasts and will either get implants or reductions to make them look "normal" and I guess "sexy" but that's probably secondary.