That really just confirms it. The one on the right is her main model, while the one on the left has an additional layer. That's how these skins work. It's the same actual model. They don't make entirely new models for skins (Lady Loki is the exception, obviously).

Once again. It's the clothes.

It's just a optical trick. The model isn't actually any slimmer or thicker. What you're seeing as thickness is really just her default's clothes.