How do you know that, though? Have you seen any rigorous scientific surveys confirming it? Or is it just a fact you absorbed from porn and pop culture and personal anecdotes?
Back in the 1800s, "everybody knew" that the Chinese where a bunch of donkey-dicked sex-addled slavering animals all desperate to cuck white men by despoiling the virtue and purity of their women. Basically the exact same stuff you see on /b/ today about black dudes. But know "everybody knows" that they have small dicks instead. It was BS then and it's BS now.
I'm not saying that average dick size couldn't vary by race. But the thing is, there's not actually any scientific evidence for it. I'd be genuinely interested if you could link to any recent studies showing otherwise.