I tried to figure out why. Was it lifestyle? More manual labor? No, even office workers weren't obese. Running was viewed as some weird western habit. Few people went to the gym. Most just went for walks for exercise.

Was it diet? Maybe. But plenty of local foods weren't exactly healthy. Mostly processed carbs. Lots of salt. Plenty of deep fried foods.

Then after eating like a local for a while I realized it. Portion size. I'd say the average meal is maybe half the portion size you'd get in the US. I'd go out to eat at a restaurant and think "I'm not that hungry", then order something anyways. But it was never a lot of food. Very common for people to have a soup dish for lunch - maybe a few strips of meat, an egg, a handful of noodles and ton of vegetables. Add it all up and it's not many calories.

It looked to me like people weren't obese because they ate less. But that was the societal norm. Sure people would gorge themselves at special events like weddings or fancy dinners out. But most of the time they were eating meals of maybe 400-500 calories each.