Westerners are generally just bigger. Even when I weighed about 55kg, Chinese people would say I was "fat." Um, no. That's not fat. I walked into stores and would be told "oh you can't fit into anything here" before I even started looking! But many Chinese girls can be quite obsessed with weight too -- even my tiny tiny students (high school girls!) say they want to lose weight, which would actually be unhealthy for them. Their standards of what counts as fat and what doesn't are a bit off. If you have any sort of body image or eating issues, China can be a bit of a head-trip. I gained weight, as women tend to do, when I had my son, and living in China makes me completely self conscious about it, and really distorts my image of myself. I know if I were back home I would still be considered to be a fairly average weight, but here, I've crossed over into "fat" territory, and it bugs me.
Keep in mind that Chinese people generally use the word "fat" and don't mean anything negative by it. One of my students is called, and answers to, "fatty" by his classmates. If you ask a student to describe a friend, they'll say "she's a bit fat" with no hint of mean-ness. They're candid about appearances -- if you're a guy, they'll ask why you're so hairy, if you have bad skin, they'll point it out as if you don't already know, and if you have a weight issue, they won't beat around any bushes or use words like "heavyset" or "large boned," nope, you're straight up fat! That takes some getting used to, but they really aren't trying to be hurtful, even though I'm sure that even heavier girls who grew up in this culture can't enjoy having their weight constantly pointed out all the time. Like I said, a head-trip.