I concur. I'm a fashion student, and pretty much no one in my class is overweight (with the exception of one or two smallfats). None of them truly buy into fat acceptance, and most are by far focused on their art.  Models are thin because clothes simply look better on thin people - that's all there is to it. Of course, the fact that thin people are much easier to sew clothes for (hams are shaped unnaturally and strangely) helps too.