I've now been in Europe for a little over a week, mostly in London, Zurich and Milan. As of yesterday afternoon, I had not seen a single person who was extremely overweight. In an hour or two walking around a U.S. city I would expect to see several such.
Yesterday evening, while I was describing my observation to my host, someone entered the restaurant we were eating at whom I would describe as extremely overweight. But that's one out of (I'm guessing) more than a thousand people I have seen over that period.
The obvious explanation is that food is somewhat more expensive in Europe than in the U.S. and incomes somewhat lower, but while obvious I do not think it can be correct. In the U.S., obesity exists all the way through the income scale—I think a little more common at the lower end. I do not believe a poor person in the U.S. can more easily afford to eat a lot than an average person in London, Zurich or Milan.
My current guess is that the explanation is cultural. Perhaps the norms against being very overweight are stronger in Europe, or perhaps some other difference explains the pattern. Readers are welcome to suggest explanations or provide additional observations.