I’ve noticed it before in some concept-to-model translations, artist goes for something very slender and then the 3D modeller bulks it up, I assume it’s just a case of the artist not having to worry about the process of turning something into a finished item while the guy doing the render has a better knowledge of what will and wont cast.
And no it's not a bad thing, I think the Sisters have found a nice balance in physical form and hold a certain grace in their poses while still looking like they can hold their own in a fight.