Sorry to break it to you but... there's a reason "average" is average... there's a distribution along a spectrum.
Also, it may surprise you to learn that porn actors are not chosen randomly from the population. They are specifically selected for their particular physical attributes.
Finally, because "average" means "average"... there is a spectrum across all nationalities. I know a couple of guys from Russia with very average dicks. So no, Russian men are not known to have, on average, larger penises than any other nationality.