is sometimes genetics. And sometimes it is because you had inadequate food. That is true of my grandmother and of one of my cousins. (My aunt fed her a no fat diet for years because when she was born she wasn’t in the lowest quartile for weight and she didn’t want her to be fat.)
I’ve never seen any scientific discussion on the fact, but it seems evident to me.
Apparently it does to John Bolton, too. In arguing against allowing luxury goods into North Korea he said, “I think, you know, the North Korean population’s been losing average height and weight over the years, and maybe this’ll be a little diet for Kim Jong Il.”