I mean you can visit Tokyo and barely notice anyone being fat while in New York there’s about 30% people being obese, even though both countries have fast food. Although Japan has less of it tending to burn off that by walking or exercise (it also reflects their lifestyle) and portion sizes regarding food & calorie intake while in US there are literally multiple fast food joints next to each other along with huge portion sizes.
It has been mentioned in the comments, but I’d like to highlight it in its own comments. The social pressure and bullying especially in middle and high school struck me as pretty high when I was there. So it is not only the food, but also the enforced perceived normal.