I dunno I've visited a lot of places and seen a lot of people and it's not like Americans are unique in how much food we eat?

I'd point to the fact that sugary/bad for you stuff tends to be cheap first and foremost. It's really easy to fill up on empty calories if you're stuffing your face with candy and fast food.

Secondarily I would point how hard it is to avoid ultra-processed garbage. It's really everywhere and everything here, even things that are "healthy" are pretty terrible usually. Unless you're eating/making everything fresh it's not easy to avoid some of this garbage and it's basically all empty calories.

I also think, far more important than gluttony, there's very little actual food education in the US. Unless you're kinda into fitness and watching what you eat, you don't really know what macros and stuff do and why things are important.

There's also food deserts, the fact that American cities are meant to be driven and basically cannot be walked/biked, and just generally sedentary lifestyles across all of America.

Genuinely I don't think that the fat countries around the world are "extra gluttons" compared to the rest of the world. It's anecdotal of course, but I often hear of Europeans visiting and gaining weight. I doubt they're suddenly becoming gluttons the moment they land in America. I think that's like maybe the 10th most important reason at best. We're all humans and while culture is different and could lead to different eating habits, I don't think that matters that much. I think Canada for example, is quite culturally similar to the US but they have an obesity rate of only 27.1% compared to America's 41.8%

I think one place where portion size might actually be a factor in retrospect is with soda. Our larges are huge and people will down those things like crazy.