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.
