I lived and worked in South Korea for a little more than a year in the early 90s and was pretty thin after my time there, but definitely not as thin as the Korean women.

We walked A LOT - up and down hills, several miles per day, no matter what the weather. Public transportation is pretty good there, but it doesn't take you straight to your garage door like in the U.S.

Fast food was only an occasional treat, although that's starting to change nowadays. We went back recently and there were many, many more fast food restaurants than there were a decade or so ago. Same thing with cakes and sodas and junk food in the grocery stores; they had several desserts, but people didn't eat them often.

We ate a lot of rice, but also a lot of low-fat options that went along with them, so maybe the total calorie consumption was pretty low. Except for white rice, we didn't get a lot of other refined carbs (well, maybe alcohol on weekends). Lots of the food were *very* spicy so made you feel more satisfied. They did eat a lot of those instant (Ramen-type) noodles which are loaded with carbs and fat, so I don't know how they managed those without gaining weight. They were mostly college-aged people, so maybe youth was on their sides.

Very few Koreans (my age) drank milk. Their kids today are drinking tons more milk and are actually getting chubbier.

Very few Koreans were obese yet, but I don't know what will happen to future generations once the junk food culture starts creeping in.