Their diets are much lower in fat. My daughter lived in Korea for a year. In the US she would never go near a McDonalds. Over there, periodically she’d get such a craving for fat she’d go to the only one in the city and get a mig mac and fries.
In one picture she was looking rather thin and I asked if she was eating enough. She said she was eating a lot. But she was having difficulty keeping her weight up on strictly Korean food. It took her a couple of months to get it back when she returned.
Interestingly, she said now that western [fast] food was coming to Korea, she could see the younger kids getting heavier.
I saw the same thing in Thailand. There were no fat older people, but quite a few skinny middle-aged people with fat kids.
It was weird. I was 5'4", about 180 lbs., and in some parts of the country I was practically Andre the Giant. My host said he was never worried about losing me in a crowd, because between my size and my (light-brown) hair, I always stood out.
I wish I'd managed a pick-up basketball game while I was there. I could have been a credible center for the first time in my life.