Is there a genetic link to their performance? Are they more athletic or build muscle easier?
Perhaps this is not the most accurate explanation out there, but I remember having this debate with a colleague a while back.
So, black people from West Africa spent their lives in open spaces on plains, so they ended up being muscular and athletic to run after animals in short bursts. Then, African-American slaves were taken from there so their descendants likewise have this genetic ability.
Black people from East Africa spent their lives in the mountains so they built slender bodies with amazing endurance skills to run for days to tire their prey - hence there are many great Kenyan and Ethiopian marathon runners.
Now, Europeans actually grew up and developed in a chillier environment so developed a higher propensity to include fat for warmth. Excessive muscle would most likely also tend to sap nutrients too quickly in such extreme environments, however a large body in general helped keep them warm and worked for cold-climate strength tasks like slicing wood. So they ended up bulkier than East Africans however without the amazing musculature of numerous West Africans. Some Europeans can be incredibly strong, however, with a mix of muscle and fat, particularly those from the coldest nations where the largest bodies tend to be advantageous.