It all depends on the way the brand is advertising its product. The model does plays an important role to promote the product but its the context that makes the difference.
I love Dove's real women campaign but it only works because its put in the right context, with the right tone of voice and the right type of models. Where as if instead of the plump models there were skinny models, the campaign would have been out of context and unsuccessful.
So it doesn't matter wether a model is skinny or plum, it matters if you put that model is in the correct environment with the correct tone of voice. Skinny models are used more simply because it is the norm, people are use to seeing it.