It is possible that the distribution range of size is wider in the black population, but that would mean that there's either a lot of tiny black guys as well or that the distribution is asymmetric (but that would mean that there are more average black guys than there are average guys of any other race since the average is still not significantly higher)
I've got a statistics exam coming up, so if I had more data I could calculate how the chances are of meeting a hung black guy compared with the chances of meeting a hung white guy. But it would suck the interestingness right out of the topic