People of color seem to perform amazing in athletics compared to other races in America (see: American football, basketball). Makes me think that Americans hyper-evolved their slaves by selective breeding. Unfortunate and extremely unethical, but maybe possible? Idk.
It's possible it's for other reasons, though. Black Americans are generally poorer than other Americans, and success in sports is a ticket out of poverty that is accessible to people in that position. It could also be a cultural thing; I doubt Finns are genetically predisposed to be exceptional drivers, but they are still wildly over-represented at the top level of motorsports for such a small population
I'd believe it if a good portion of black americans I've seen are tall and muscular, but I'd say their proportions are similar to their white american counterparts, i.e. a spectrum.
I think they just want it more, and its one of the few paths to success they have.
That actually can be explained by the slaves that used to live in the farmhouses. They were treated as an elevated status and allowed to eat more. Think like the "aunts" for the children; caretakers.
But they were exceptions, not the majority who were slaves/workers. You'd still expect a heavy selection bias for good body attributes
if you sampled them at random, assuming OP's hypothesis is true.
I heard that a similar breeding program happened in some African cultures back in the day. Like you could only breed if you were an alpha hunter or somesuch.