She isn't fat - she has thick, muscular thighs, like an American super heroine, or an action game fighter like Chun Li. I think that's rather neat.
Edit: As for the current coloration, they're using high contrast colors to differentiate the skin tones from the rest of the detail work. It's probably easier to ensure you have the right facial shapes and shadows for a humanoid when you work in a human skin color. Once they've got all the sculpting done, they can switch to changing the cast of the skin color to match the concept artwork. That'll probably be one of the final steps.
Edited, Jan 22nd 2013 3:32pm by catwho