Besides the fashion industry aiming at presenting a waifishly thin image, in more stylized artwork, its a lot easier to draw thin people. I know because I've drawn both. Lolita as a style of art pretty much requires it, as the girls aren't presenting themselves as little girls as much as they are porcelain dolls. (IMO)
When cosplaying, I think one's body does play into it. This can be unfortunate for thicker people, darker skinned people, ect because there's just not really many characters to pick from.
