The Japanese just eat less calories. There isn't anything particularly fancy about it, it's just less overall, be it rice, donuts, or fluffy Hokkaido milk bread.

> The answer is not a simple yes or no. While there are many factors that influence individual diets, the overall Japanese diet is generally lower in calories than that seen in some Western countries. On average, Japanese adults consume 2,150-2,900 calories per day, while the average American adult consumes 2,742-3,657 calories every day.[1]

Assuming the middles of those ranges that's like 500-1000 calories less than your average American. This does not mean they're more "fit" either, though things like pull-ups and 5K runs are easier when you're skinny vs. 50lbs overweight due to excess adipose tissue.

[1] https://www.thedonutwhole.com/do-japanese-eat-less-calories/....