If they knew the origins of this claim go back to slavers, they might not be in such a hurry to keep it going. In boot camp we only had a few guys black and white with certified monsters in their skivvies. And only a few really small fellas. The vast majority were virtually the same, average.
It's also kind of pitiful that a group of people feels any need to brag up a generic physical attribute and use it as a point for racist chiding of the others. But that's the America we live in at the moment. As education improves, maybe we'll see more class in our ranks. Instead of people going on and on about their dicks. To the OP, a better question might be "why are they always grabbing their dick in public?" (Sorry, I don't say "penis". It's a small-sounding, wimpy word that no man should suffer with)