Actually, you may not be far off in that assessment, Princess; I remember reading a study on penis size that stated - on average - most people are packing about equal, the only difference is black men tend to visit the extremely large more often than other races. Sickle Cell Anemia is related to priapism, a state in which prolonged erections lead to phalloplasty, an enlargening of the penis - whether there's any connection between priapism and the prevalence of large penises in the black community, I don't know, but there seems to be evidence that supports black men being ridiculously endowed more often than other races.
Oh, and yes, I agree with the shower vs grower thing - it would be advantageously adaptive for men in colder climates to have smaller flaccids, as it would conserve heat; the same can be inverted for warmer climates.