First, genetics. Vietnamese are small framed compared to other ethnicity. Asians, also, are generally smaller than Westerners.
Diets. In Vietnam it is cheaper to eat healthy than to eat unhealthy. Vegs and fruits are available and cheap. The cuisine also consists of a lot of vegs. It is standard for every meal to have three dishes, one of which is vegs. Desserts aren’t cake, but fruits.
Poverty. It used to be that way in before 80s (people were poorer, work more physically, and overall thinner back then). But this isn’t the case now.
Fat shaming is still prevalent. If one is fat, especially women, they will get look and words from others. Not everyone is insensitive, but the notion that “fat isn’t good” is very obvious. No one would tell you “you’re beautiful being fat”. Only the other way around.