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.
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."
Just needs to be a walking friendly city. You will lose weight in eg NYC or Seattle
That's evidence of environmental factors.