I've heard the latter theory at least a couple times about US Navy SEALs.
The first time, it was a retired SEAL I knew (well over 6', and a brick wall) who one day out of the blue said something like, "You shouldn't feel bad about being short. The best SEAL I knew was a short guy, and he could kick my ass."
Later, I heard a similar anecdote in a speech: https://www.youtube.com/watch?v=pxBQLFLei70&t=440s