It comes from an innate cultural understanding that fatness comes from laziness and lack of disciple. The look of utter disgust some Chinese and Japanese can summon up when seeing a fatty is an awesome thing to behold.  As my sifu says (in the context of intense stance training) "If your legs start shaking and you collapse, then you have weak legs. I can help you make them stronger. But if you give in and stand up, then you have a weak mind. I cannot help you with that." It's a culture where excuses and lack of personal responsibility are not accepted and laziness and greed are quite rightly seen as shameful.