Africans are larger, because their women are larger. Hard to hit the sweet spot on something that big, without being equipped for the job.
Negative
Africans are longer because of an evolutionary causation that resulted from a super hostile environment.
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With multiple males breeding there same female, the longer member was able to make deposit closer to the cervix and more likely fertilizer the egg.
Therefore passing on the gene of the longer member.
Society's that formed civilizations earliest promoted monogamy earlier as well.
Africa remained hunter gatherer and uncivilized well into the 19th century.
That point being made, no connection between size and fertility has ever been made.
I myself have twin daughters.