Now there's a post I can't help but respond to. You've definitely raised an interesting point. The unfortunate truth is that nearly 65% of Americans can be classified as overweight. I imagine that many other Western countries are fast approaching that point also. So, doing the simple math, let's just say that six out of ten ESL teachers, be they men or women, are overweight. But your question is more direct and suggests that you have observed that more than six out of ten female ESL teachers in China are overweight. Further, you wonder what lures them to China. So do I. I also wonder why they don't lose weight in China. In America I've always maintained a certain weight that I find acceptable, but my problem is the opposite; that is, I have to work hard to keep the weight. However, in China I can't keep my weight up, and it only takes a month or two before I lose about 20 pounds and most people think I'm too thin - for an American. In some ways this points to the obvious truth that the concept of correct weight is relative to cultural norms. So some Western women that you consider to be "heavily built," for example, may be considered only Rubenesque (not sure about that spelling)in other cultures.
The simple answer of course is that those women you refer to, cultural definitions put aside, have an unhealthy life style. That is, they eat too much junk food, drink too much alcohol, avoid exercise and generally just eat way too much food of all kinds. But, as most health experts will tell you, the answer is not always that simple.
Well anyway, I know this doesn't answer your question. And in fact I'm as confused as you are by the phenomenom. Thanks though for making me think.
I'm looking forward to other postings that address this issue.
By the way, I just spent 8 weeks in Shandong Province and was shocked to discover that many Chinese women, especially the younger ones, fit into the above mold of athletic builds or are decidedly overweight. I was glad to return to the South where I don't feel so diminuative.