Black from certain African countries have genetics for lower fat across their body which could perhaps give the illusion of being more muscular.
Some studies conclude that blacks have higher testosterone levels than others races as well as a denser bone structure and maybe higher-twitch muscle fibers?
Caucasians, especially Europeans can have up to 6% Neanderthal DNA, slightly less for Asians.
The reason being that a different migratory groups of humans spread out of north Africa at different times.
Central African blacks were not part of the same migratory group which spread across the rest of the world.
Other races all share a common ancestor with a single female from north Africa (apparently, saw this on a BBC documentary haha)