It's because she's based on KOF Destiny and to be honest Mai is never portrayed as toned or muscular...she's always just been busty and thick around the butt and thighs......i do agree though they are thin.
From here though they look fine.
I still think this is the best Mai so far but Kinnetiquettes are basing theirs on Artgerm Lau art and HMO have yet to show their hand so the wait is still on...