First of all, the studies quoted all give an average erect length of about 5.3 inches and a standard deviation of about one inch (ie. 68% of men in each study have erections between, say, 4.3 and 6.3 inches.). The Kinsey study is the larger group of subjects but the measures were reported by self measurement so probably contain a degree of "rounding - up" or slight exaggeration.
I have not seen very many African or African American men in my clinical practice but my impression has been that African American men have larger penises.
Elsewhere on LPSG I have speculated that slavery involved people being treated like farm stock and many slave owners bred for strong bodies and large penises. There are some accounts of the slave trade period that support this speculation.