I think what people fail to take into consideration is the reason the big dick black guy rumor is alive is because women are choosing to have sex with the black guys who are the clear alpha males of the race. They aren't attracted to the fat, ugly or weak black guys, just the top physical specimens.
That is the reason they like black guys; it's all physical attraction. So obviously the blacks of top physical caliber are going to have the largest genitals. Of course there are tons of black guys with small penises, but I imagine a good number of them are less physically developed than more physically developed, in general.
So white black cock sluts are deliberately screening black men to get to the ones with the big black dicks. What other race do women do this for to this extent? You mention a black guy in the Western world, what is the first thing that pops into your mind? Big dick. So this is how the rumor is spread.
Again, this is another case of my point that analyzing averages is useless because only a certain percentage of men are sexually active with more than one woman. These statistics only give you the average penis size of all men, not the average penis size that the average woman is getting.
In other words, the average dick size could be 6 inches, but the average penis size sexually active women are used to getting could be 7 inches, due to the fact only a certain % of men are having regular sex with multiple cock sluts. It's my belief that this % of men are above average to well above average in penis size.
If you think about evolution, my theory is very plausible.