This cultural-differences explanation certainly accounts for why anorexia nervosa is mainly a disease of higher-income girls and young women. Since obese men are less stigmatized, it may also explain why wealthier men are not that much less obese than poorer men.
Still, if thinness really becomes the ideal in every affluent culture--the way plumpness is in poor societies--it's curious. Is this simply the nature of status--the rich will always find a way to distinguish themselves from the poor? Or does richness breed an instinct for longevity? Whatever the case, I'll know India has finally risen from Third World status when I go back to visit my relatives and the first few words they say are not "Eat, eat, skinny boy."