First of all, I don't believe in human races. There is too much of a continuum, and too little genetic difference, between ascribed "races." The number of genetic differences responsible for the outward differences between whites, Asians, blacks, etc. are much fewer in number than the average number of genetic differences between any two randomly-selected unrelated individuals of the same "race." That being said, I do believe in population genetics.
Since we evolved in Africa, our largest gene pool lies there. Thus within the black population, which covers the entire continent of Africa and some other areas of the world, there is more variation. Any population from anywhere else in the world descended from groups that left Africa in one or more of the waves out of the continent. As such, any other population will have less genetic variation than that which lies within the continent of Africa. A simple way of explaining it is to think of Africa as a big can of different marbles, some types rare and others more common. If you think of each wave out of Africa as grabbing a handful of marbles at random and replicating that which is contained within the handful, you'll be able to understand that the handful most likely won't contain every different marble in the same proportions, and will likely be missing some marbles (especially rare ones from the main group).
This idea came to light as a reason why organ transplants seemed to have somewhat lower rates of success among black people than among white people. What was discovered was that scientists identifying minor tissue types (besides the ABO and Rh factors) were testing for only those discovered among white Europeans. It was later realized that there existed some additional minor tissue types among black Africans which did not exist among the white Europeans because individuals with them did not produce descendants among white Europeans. Either no individuals left Africa with these tissue types, or they did so only in small numbers and for whatever reason, they were "out-bred." Thus organ transplant tissue typing wasn't testing for these additional tissue types and there were previously unknown mismatches among blacks.
What does this mean with regards to penis size? Well, if Africa contains more genetic variation than any other continent, then it stands to reason that the bell curve for penis size, among other traits, is potentially wider than among any other continent's peoples. As such, there will be more variation from small to large, and if people were early on "astounded" by some extraordinary sizes among black Africans, this remained in their minds to foster "the myth." Encountering those more in the average or even small range might have been more easily dismissed as "nothing special" and not remembered, while the big ones would stick in the brain, thus beginning the perception that blacks are all big.
What does this say about the idea that Asians are small? Well, from what I've come to understand about how the world was peopled, China was overtaken at an early point in its history by one group of people which then grew in number to fill the land and largely prevented further immigration from other groups. Thus China became rather homogeneous for such a large area of land, and over the ages spread outward to other areas of SE Asia, generally overtaking or at least "genetically swamping" previous populations. As a result, the Asian bell curve is more narrow, with less of a difference between "small" and "large", and with more of a rarity for "large", engendered the impression that Asians were "small" (when in actuality, they were more numerously average, and generally the same "average" as for just about everybody else).
