Well here’s an interesting post comparing average heights in 1912 and 2012.
If Americans are on average getting shorter, which I doubt, I wouldn’t be a bit surprised if it was related to immigration.
I live in a neighborhood where there are a lot of Central Americans, and many of them are really short.
Forgot the link. Sorry.
http://ahundredyearsago.com/2012/02/06/average-height-for-males-and-females-in-1912-and-2012/