I've read and concluded that different penis and testicle sizes complement different sexual strategies.
One thing to keep in mind here, is that a man will have a large penis for a good reason. If it didn't, it wouldn't be there, because a large penis takes up a lot of resources. A larger pair of genitals uses significantly more resources mainly because it produces more sperm, which is very energy intensive. So the gain a man gets from a large penis must outweigh the cost of all of the resources that go into it.
In fact, often the resources that could have gone to a large package, might go to something else that's useful, like extra brain tissue. Both brains and genitals are very resource intensive, and as a result, appear to vie for resources. So as a result, often a person who has smaller genitals will have larger brains, where a person with larger genitals will have smaller brains. This has in fact been observed in bats. Here is an
article about it. Different variations of genital and brain sizes represent different reproductive strategies.
Male bats with the largest genitals will try to inseminate as many females as possible. Because it has large genitals, it will have more sperm, which will make it better at doing this. It also makes it better equipped at impregnating females that already have sperm in their cervixes (or whatever bats have), because a larger army will be better fighting the opposing team and enabling their own team to get the egg.
Male bats with the largest brains will do the opposite. They will find a mate and stick to her and use their brainpower to prevent her from cuckolding him.
He will not go exploring because he doesn't have the equipment to make it worthwhile. It is important to note, however, that the brain size will probably help not only in preventing cuckoldery, but also other things, like identifying other bats, communicating and finding or sharing food.
How successful each strategy is depends on the ratio of each strategy in a population (keep in mind, that those two strategies I described are extreme examples--there are likely to be intermediate strategies as well, but those are the two main ones). If you are a large genital bat, you have the ability to possibly impregnate LOTS of female bats. But if there are lots of others like you, you might lose in many sperm wars. You might have even lost in the sperm wars with your own mate because you were gone attemping to inseminate other females, while she cuckolded you.
In that case, a large brained bat would have the advantage, because he would not allow himself to be cuckolded. He would have a ticket to the next generation, although he wouldn't have the same reproductive potential as the other larger genitaled bats.
The main thing to get from this is that the large genitals in males must absolutely justify themselves, because they require so much in terms of resources. Now could the costs and benefits of those different brain/genital configurations possibly
vary from geographic location to another? I think the answer is very likely to be, yes.
It is speculated by scientists studying intelligence that the reasons for the different IQs for different races may be because of differences in climate. Colder and harsher climates are more challenging and often reward higher intelligence because it helped humans prevent themselves from freezing and starving (probably because it increased their ingenuity).
This increased the selection for bigger brains and because brains and genitals are so resource-intensive, their genitals shrank because higher intelligence was more important. If you were a woman at that time, it wouldn't matter that you liked big dick, because a large-brained man would be better equipped to protect you and bring you meat.
Now consider: Negoid males have the largest genitals of all races. They also have the smallest brains and the lowest IQs.
Mongoloid males have the smallest genitals of all races. They have the largest brains and the highest IQ.
Caucasian males have intermediate-sized genitals. They have intermediate-sized brains and IQs between those of blacks and East Asians.