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.
Tokyo’s obesity is rising, as is youth obesity in general. More shit food, less manual labor, shorter work hours, shorter work week (school and work used to be 6 days a week and work days in particular could be (and sometimes still are) quite long; this only changed recently, in the '90s IIRC). I suspect it will continue to grow.
The people who work desk jobs and stay fit move a lot, eat smaller portions, eat less bad food, and drink few calories. People are consuming more tea and coffee than sodas and other similar drinks. In urban areas, people are often easily hitting 10k+ steps a day just going to and from work/school/errands. Many of the fitter people who have time and money also do something like kickboxing, dance, etc. (though that tends to stop once they have kids).
A lot of this is anecdotal from someone over a decade in Japan.
Edit: Ah, also societal pressure to fit in, particularly worse for women as is often the case. People will not be shy to call you fat(ter), though it can also be because of a genuine worry about your health if they’re your friends.
I read a lot of manga, and the amount of references I see to going to school on a Saturday or leaving the school building at 7pm. It is unreal to think about.
Leaving later can be because of 部活 which is often translated as ‘clubs’ but I might call them extracurricular activities coming from the US. Dance, table tennis, soccer, baseball, chess, electronics, swimming, etc.
Growing up in the US, I did band, theater, sports, etc. and it wasn’t uncommon to be leaving school fairly late because of it (theater especially).