I think this is more just a 'woman pose' than a fat woman thing, if that makes sense. Commonly seen in exasperated mothers and sassy teenagers of all sizes, hah. Something about the combination of it being an easy resting place for hands and showing confidence/strength. As far as art/advertisements go, when they are aimed at men they are often attempting to emphasis the curve of the hips, like said above.
I'm not sure there has to be a benefit to standing a certain way though. =p