Chinese and Mongolian men (I can only speak for Chinese and Mongolian men here when it comes to Asians) are proportionate to their body size. To most foreign women, we would think them on average to be a little shorter in length than those men of European descent because those of European decent are much taller and different body structure. Chinese are shorter in height so a six inch erect penis on a five foot man would look kind of ridiculous and totally out of proportion. Mongolian men on the other hand, again are proportionate, but I do find them to be much thicker than their Chinese counterparts simply because Mongolians are more "manly" looking (please read this as they have defined muscles and meat on their bones).
This is not based on any scientific fact, just what I have experienced living here in China. And this is only on average. Some of the Chinese men I have been with are more girthier than their foreign counter parts and longer, and some are much smaller. But then again I have been with Western men who were no bigger than AAA batteries fully erect.
It would be similar to a Chinese woman having American size C cup... It would be like Celine Dion having a DDD cup. Totally out of proportion for her body.