Fat is never regarded as 'attractive' at least in my native country (I'm from Vietnam. I believe Vietnam has a very similar culture compared to China.)
The most intuitive way to explain why 'being fatter' is regarded as a complement is this: People often say 'you look different' when they see each other after a long time. Now of course there are two way provided the person looks relatively okay, that is being fatter or thinner. However, thinner might imply that he/she was sick or something that caused him/her to lose weight. So saying 'you look fatter' is a way of saying you're doing well. In fact, Asian people tend to be thinner -- even in the US -- gttp://www.statehealthfacts.org/comparebar.jsp?ind=91&cat=2 . So it's _understandable_ why such Asian countries don't consider 'getting fat' as an offensive thing. Also it's more like a figure of speech now.
The most intuitive way to explain why 'being fatter' is regarded as a complement is this: People often say 'you look different' when they see each other after a long time. Now of course there are two way provided the person looks relatively okay, that is being fatter or thinner. However, thinner might imply that he/she was sick or something that caused him/her to lose weight. So saying 'you look fatter' is a way of saying you're doing well. In fact, Asian people tend to be thinner -- even in the US -- gttp://www.statehealthfacts.org/comparebar.jsp?ind=91&cat=2 . So it's _understandable_ why such Asian countries don't consider 'getting fat' as an offensive thing. Also it's more like a figure of speech now.