I think it comes down to \"Compensation\".....We all, OK most of us, compensate to make up for what we \"believe\" we are lacking. A short nerdy guy tries to be good at something that his genetic make up did not give him a head start with. So, may be he becomes very good with computers or video games and such. I hope you know what I am trying to say here. If a guy is hung well and is handsome, he may not feel the need to be top of his class, so he may not be that smart or well educated, because he can lean on his good looks and his big member. This does not apply to every one of course, but it is a generalization.
For example, I am average height by US standards and I am tall by Asian standards. I feel a bit short in the US, so I try to be smarter, dress better and be more social to compensate for my height....Should I be thinking this way? well, I do not think I should, but our social standards and programming tell us taller is better just like bigger is better. It is not right, but it is real. Statistics \"DO\" confirm that taller guys get hired more often!
As I understand it, generally speaking, penis size is based on genetics. When ones is created, some DNA are turned on and off to form the baby, if the penis gene is turned on a certain amount of time and in a certain way, his penis will be a certain size. The same goes for height, color of eyes and so on. So, we all have all the traits in our system right now, but because some genes were turned on and off differently when we were created we all look different.