Might be a marginal size difference, but the numbers are often exaggerated.

http://onlinelibrary.wiley.com/doi/10.1111/bju.13010/full

From a real consistent study they conclude " Lynn [31] suggest that penis length and girth are greatest in Negroids (sub-Saharan Africans), intermediate in Caucasoids (Europeans, South Asians and North African), and smallest in Mongoloids (East Asians), but this is based upon studies that did not meet our present inclusion and exclusion criteria. The greatest proportion of the participants in the present meta-analysis were Caucasoids. There was only one study of 320 men in Negroids and two studies of 445 men in Mongoloids. There are no indications of differences in racial variability in our present study, e.g. the study from Nigeria was not a positive outlier."

Many sources likely choose to use the one that simply did things wrong or weren't consistent in their studies.

This next bit is just a personal guess. But it seems to me that since black dudes are often tall and skinny from these countries that it becomes easy to measure. While white dudes from these countries are more likely to be fat which hides the true length, and lastly asians are shorter and pudgier than black guys so they get the shortest end of the stick.

If you make a thread about penis you should expect people to dick around.