I always wondered about that too. I am of 100% American Southern descent with deep English, Scottish and some Scandinavian roots. I am either 6’1" or 6"2 depending on the time of day that you measure me. I have always been slightly taller than average but it wasn’t really notable until I moved to the Northeast. I feel like an absolute giant compared to most people here no matter where their ancestors came from (largeky Southern European). However, even the English, Irish and black people aren’t nearly as big as they are in the South. Every time I go back home, I am immediately struck by the fact that I am far from the tallest person around for the first time in a long time. Some of them are just fat and moderately tall but certainly not all of them. The younger males, both black and white just tend to be unusually big compared to people in Boston or NYC.
I have heard the dietary explanation but it seems incomplete to me. We didn’t eat especially nutritiously when I was growing up but we still grew a nice crop of extremely tall and statuesque people from several different ethnic groups. Only the recent hispanic and Asian immigrants were small as a whole.
I would also love to see stats on height differences among different ethnic groups in the U.S. and exactly where they are coming from because it may help answer the question much better.