Female sex workers deal with hundreds of the male genitalia. Long experience leads to thousands or more. From what many told me, the size and shape varies a great deal. It’s beyond men’s imagination.
In some senses, one of the jobs that is the most familiar with the penis might be sex workers.
Not a small number of them told me the same story: If a Japanese guy has a big dick, he is very likely to be short, thin and have a muscular build.
An ex-nurse worker said that it has a medical background.
First of all, the size of the heart is almost the same regardless of the height or body type, whether a guy is 5 or 7 inches tall, and extremely thin or fat. The heart is designed to pump blood around the body. With the same function, less workload should come with more effectiveness. Also, blood concentration causes erection. In other words, the smaller body makes it easier for blood to concentrate on the penis. The heart function uses the muscle throughout the body. Needless to say, more muscle means better circulation.
The above is the reason why short and muscular guys tend to have a big dick.
I don’t know about non-Japanese men. According to surveys, Asians including Japanese have a smaller one compared to Africans in particular.
Incidentally, foreign women I know told me that the Japanese penis is harder than other people’s. It could be national characteristics, or because of the diet.