If Asians usually underperform, how come they are so successful in reproduction?
And BTW, there's no real study that points to a difference in penis size that's linked to ethnicity.
The hypothesis was proposed by a racist psychologist (not a medic or anything) to propagate the idea that black people are focused only on sexual reproduction and easterners are only focused on intelligence, leaving white people to be the perfect race in the "goldilocks zone" of balancing the need for sexual reproduction and intelligence.
The current "evidence" you can find usually falls into one of these categories:
1. completely made up - there are websites that literally cite each other as sources, and claim they couldn't be any more specific as they were sensitive data;
2. self-reported - there are many websites that have allowed random internet users to self report their penis sizes, and anyone can type in anything, claiming to be from any nation;
3. unrepresentative - many sources from more scholarly articles cite sources that were in no way, shape, or form from random sampling. Examples were "a diary of a French war doctor during WWI" and "data from 30 men at the Afghan sexual health clinic".
And as a racial hypothesis, the make-up data do not match what we know about races in different countries at all.
To give you several examples:
a. Caribbean countries are mostly populated by black people, and many of the countries on the continents have a significant black population. Yet, in many "studies", they don't have particularly long penises, presumably because the racists who made up the numbers forgot the fact that those countries are around just as black as many African countries are;
b. the United States is usually listed alongside European countries, despite the fact that it has a lot more black people among their population, compared to say, Ireland and Poland;
c. Mexico is somehow listed with longer penises compared to all other Latin American countries. The interesting thing about Mexico's ethnic makeup is the fact that it's mostly Native American-white mixed, with a very significant portion of Native Americans. So why is that a problem for this hypothesis? Native Americans are Mongols, just like East Asians are. So, if the hypothesis is true, Mexico would have shorter penises if anything, especially compared to Brazil and Venezuela and Colombia (a lot more black people), as well as nations such as Argentina (practically all white). But of course the racists who made up the numbers didn't realize Native Americans are by blood the same race as East Asians are.
d. East Asians are not all from the same evolutionary origin. In regions such as southern China, Macau, and Hong Kong, the Han Chinese population was mixed heavily with peoples coming from Indochina. People in northern China, as well as Korea and Japan, came from an entirely different evolutionary route in the north.
e. Somehow Australia has reportedly longer penises despite the fact that it's mostly white, with East Asians being its biggest immigration group after that.
There is no evolutionary reason why certain groups would develop longer penises compared to others. There isn't a limit on how people could evolve - to say "blacks evolved with longer penises so they ran out of evolutionary energy to be smart" is not scientific. And of course, if the hypothesis were correct, which there's no evidence suggesting it is, then it'd simply prove the irrelevance of penis length, as China and India are currently the most populated states on Earth.