I dunno about China and Sri Lanka specifically, but in some cultures, being fat is seen as attractive. This is probably a signaling mechanism, being able to afford additional food and not needing to perform manual labor means that you are more successful. Being fat isn't so attractive in the West because having a sedentary job and access to excessive fat and carbs doesn't really signal anything much.
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.
