People that move out of the USA generally lose weight. Especially if they move to a country with snaller portions and more walking. People that move to the USA generally gain weight. Evidence that it's lifestyle.
An environment that physically makes you more sedentary as, outside of a couple of cities, many things that would be sensibly done through walking in other countries involve driving.
You can easily tell this is the case by seeing where the obesity is less prevalent
I would suggest that you look at food labels of "equivalent" products on both sides of the Atlantic. US packaged foods have a lot more sugar (and general calories) than those in Europe, even when they are the "same."