Your observation has its parallels in Europe, too. Maybe not to the same extent in quantity but certainly in quality. Obesity rates are up here in Germany, too. Unlike Nadar, I'd blame sedentism and overfeeding in equal parts. But that's just a personal guess and observation.

The proportion of junk food on people's plates is definitely a strong factor. My wife and I have a little game that we play at the Aldi checkout. "Guess the size of the person from the food they put on the counter". if it's mainly frozen pizza, chips , and all the other packaged stuff, the shirt sized is always XXXL. If it's mainly fresh stuff, you'll have one of those rare slim persons ahead of you.