And getting an actress to put on 20-40 lbs. isn't exactly the easiest thing to do. As I mentioned in the "House of Finarfin" thread, Jewel Staite gained 15-20 lbs. for her role as Kaylee on Firefly, and it was considered rather shocking. Kaylee isn't even what I would consider stocky.Right... I mean I can find short actresses easily. Mila Kunis for example. I guess you could add stockiness but that would feel like cheating.
Also, I would prefer that she be solid rather than just fat. Fat girls aren't _that_ hard to find in Hollywood, but they hardly bring to mind someone that would be capable in combat.
EDIT: DISCLAIMER: I am not using fat as a derogatory here.