Just needs to be a walking friendly city. You will lose weight in eg NYC or Seattle

I'm in Seattle, and I can tell you that foreigners don't move here and lose weight.

That's evidence of environmental factors.

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

Maybe peer and societal influences too?

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."

I visited us in summer, from Italy. Bread was so sugary. Cakes were sugary. Everything was much more sugary than here