False on the muscle thing.
After I got into lifting, I found out the reason this myth exists is largely due to contrast. Why do you think bodybuilders get so bronzed before going on stage? Because light hits the muscles and pops more under a dark appearance. Muscles look more big and cut just because of the way the light hits them. Plenty of huge guys are not black, and actually most elite strong men tend to be white guys from Europe.
The over-representation of black athletes in American sports causes too many people to ignorantly assume that all blacks will get some muscle or athletic edge over everyone else. When in reality, its one subset of blacks in the Americas that excel (west african descent)...and there are tons of blacks of different bloodlines who wont perform the same way. Also, black over-representation in 2 of Americas major sports can be explained sociologically as well.
Just look what happened to baseball...where its hardly black anymore. If you have a community where excelling in certain sports is promoted as a major way to succeed at life...youll see over-representation in a particular sport if other communities arent pushing their youth in the same manner. What Im saying is that certain communities push their youth to do very well in sports, more so than other communities. Its why Samoans are seen as so great at football and greatly over-represented in the NFL...despite American Samoa's small population of less than 70,000. And Im sure no one would say those guys are naturally just better than blacks or whites at football. American football is just a big part of their culture....like the way they push some of their youth is huge.
http://en.wikipedia.org/wiki/American_football_in_American_Samoa
Anyways, Im not saying there exist no physical difference between races...but its vastly overstated by people. I see plenty of huge dudes walking around daily that arent black. And I see tons, and I mean tons, of skinny black guys. I just think people have bought into the "big black guy" idea so much, that they notice a big dude more if he happens to be black.