Funny enough, if you chopped off the fattest 5-6 states, the USA would compare more favorably.
Sources: https://en.wikipedia.org/wiki/List_of_countries_by_obesity_r...
https://ourworldindata.org/obesity
https://worldpopulationreview.com/country-rankings/obesity-r...
That is self evident in social behavior. Your theory is just an easily trotted out bogeyman.
The main difference is availability and price, especially the latter. A decent lunch near my office in my home country was 20-30 bucks (converted in USD), and I made X/year. Now I make 3-4+ times X, and you can get more food than I can eat at shit like Panda Express for 6-8 bucks. So vs my income, it's almost 10 times cheaper.
If it wasn't for health concerns, it no longer made sense for me to cook. So I just had delivery/takeout almost every day after I moved (switching country is stressful, so I was happy to get all the time back I could get). Then the health issues cropped up. Now I'm trying to dial it back, but its tough.
I know a _lot_ of people who had similar things happened to them after they moved here, and anecdotally again, one of them moved back and started losing weight super fast, without really trying.