In response to my observation that I hadn't encountered as many very heavy Americans as I thought I would on return from three years away:
"My wife (who is Australian and green card-holding) and I ( who was born in England but am now naturalized) were married this summer in Washington DC. Naturally her Australian family and some English relations of mine came to the wedding; for all of them it was their first time in the States and they combined their trip to DC with the usual American tourist visits (Disneyland, NYC, etc). On separate occasions both the Australians and the English said to me more or less what you wrote: "There are not as many fat people here as we expected".
"As far as I understand it the rest of the Anglophone world is not so far behind the US in terms of obesity rates so maybe their stereotypes about America haven't caught up with the new, fatter reality in their own countries..."