Confusion related to effective population size
That is a fantastic question and in my opinion a really important question in context. The Ne of Africa will be much bigger than the European population, however I'm less sure about the West African population which gave rise to the Afro-American population. I suspect it is still large because of the "out of Africa" theory. 

My advice is to start by taking a complete look at the entire subject, from Tenesa et al (2007).  Ne is a really important parameter in any population study and extremely useful. I have feeling this could be complex, but I cannot hypotheses any further because I don't do human genetics (except for immunology).