Hi Mizu

I don't know a whole bunch about the subject in particular, i studied exercise and physiology but when it comes to cross cultures its a little harder to say without being in the country in particular for a certain amount of time. From what i know, since Japanese people in general are quite slim due to their diet and they frequently get some sort of exercise (iv even seen the elderly doing proper gardening and also cycling). But it's also noted that the overweight population of Japan is beginning to rise (it's small percentage but its there), the Government have taken note regarding this and actively promote some exercise time during work etc.

I would recommend checking a youtube user called 'ChemistinJapan'. He's someone who has been to Japan frequently and possibly going to live there and is also overweight. He does a video about being overweight in Japan and gives his more personal opinion regarding it.