I've often said that runway models are skinny for the simple reason that they are all skinny, and therefore are a consistent fit for the clothes. Runway shows are to display CLOTHES, not people. The target audience doesn't care about the   3242  models, as long as they all look similar, because that makes it easier to compare the clothes. That's also why some of the outfits at fashion shows look fucked up. It's so the model can show underlying clothes, or to show part of a piece that would otherwise be invisible to the audience. They usually all show a resting bitch face for the same reason: its neutral and consistent.