It's numbers people, that lame thing we refer to as math. Having a bigger dick isn't so much relatated to your genetic nationality as the specific gene sequencing from your parents, coming out of a vagina in the US vs France or Camaroon. Those odds of being borne with the proper sequencing already hit a rather low number, being born with a working penis 8"-XXX".
That being said any apparent drastic difference based nationality or for otherwise ethnicity based arguments become baised because your population is larger and therefore those low odds we spoke of earlier become more apparent, say 1 out of 500 Americans is over 7 & 1/2", now multiply that math to fit the total population. Suddenly that number gets a little bigger. Its the same reason ethnicity gets confused for penis size, again look into the actual math when you start looking into the total number of people.