That's what surprised me the most visiting Japan. All the skinny people in Japan eating fried chicken, fried mashed potatoes, fried everything.
The only thing I could make out for why they were skinny was insane levels of portion control. Since they definitely didn't have time to do sports.
[0] https://en.wikipedia.org/wiki/French_tacos
When you say they seem to live off this diet, what are you basing that on?