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.

Japan has walkable cities

Food and culture. Those countries have nothing in common for that.

portion sizes ... and calories

It's the only scientific way to lose weight.

It feels like lots of people here in the US have just given up.

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.

I find it hard to believe any US city has obesity rate as low as 30%. Maybe grossly obese at 30%

When we were in Tokyo and SE Asia, we lost weight. The food is lighter, healthier, smaller proportions. The rice is not handed out like some sort of endless buffet. You get a cup of rice or whatever the measurement. That's it. No additional servings of rice. Andrew Weil did a whole description of how cultures who's staple foods (rice, corn, grains, taro...) have become more finely processed become more obese, across the board, as a culture. When it becomes fine powdery flour, it breaks down into sugar way faster. You're not supposed to eat that shit for good reasons. Lifestyle plays a big part but cutting out all that junk food is critical.

Hara hachi bu

Bread, pasta, Potatoes, hIgh fructose corn syrup. versus Seafood, fish, and fresh fruits and vegetables.