I've recently returned from 2 1/2 months in Asia.
You're absolutely correct, there are few even moderate fatties and none of the huge people we commonly see here in the States. I think I saw only one local I would consider fat by American standards.
Being overweight myself (but not in the freight-scale category), I was embarrassed to hear the truly amazed comments by the locals about my weight and that of other Westerners (I speak one of the local languages). It shamed me into losing about 30 pounds, which I wanted to do anyway.
My theory is this, shoot it down if you like: America has become a very "closed door" society. We get in the car, go to work, get in the car, come home, close the door, and watch TV or videos or use the computer. Look in your own neighborhood - there are few opportunities to walk to a local cafe, which is certainly the rule in Europe and Asia. Eating your meals at one of dozens of cafes or street stands is an everyday occurance in most Asian countries. Since eating is somewhat of a social activity as well, sitting alone in your house and binging on snacks and huge meals would be considered positively weird there.
Isolation, loneliness, suburbia, the most demanding worklife of the industrialized nations, eating as private pleasure, etc. All of this combines to create a nation of fatties.