I think some factors also impinge on the what DM constitues as "fat" as Hollywood, for one, has a rather different definition to the rest of us...
Also, a point that's just occurred to me; if she's going on adventures, doesn't that mean she'll be losing weight fairly sharpish if she's actually really obese anyway? 'Cos there's no way that stomping round in full plate and swinging a sword isn't good exercise!