I have a theory that I don't think I could ever work to prove fully, but it's fleshed out enough that I can now share the main details:
I think the quality of guys' butts (let's say from puberty to 25, so we leave out the people who got a good butt as they got older) correlates directly to the size of those same guys' dicks. Let's lay out a few facts here:
1) Penis size is largely hereditary but is most affected by boosts in testosterone directly before, during, and after puberty
2) Compound lifting mildly to moderately increases testosterone
3) What is the compound lift that is most beneficial to the body as a whole (arguably)? The squat.
4) What muscle do you contract at the height of the squat once your quads are disengaged? The butt (if you're using proper form, which you should do, always)
5) I have an average butt. I don't focus on hypertrophy of the butt in the gym because I worry it would take away from all the focus i've put into other muscle groups if it got too good. I have, at 20% bodyfat, an average penis. Like literally textbook united states average. (At low body fat, it actually looks pretty decent on my body btw)
Now for the theory:
The flexing of the gluteus to achieve a pump at the height of a squat or a lunge or a hip bridge forces blood (because that's what a pump is, literally blood rushing) to the middle of the body, where it can either provide to the flexing muscles or what's around them.
What's literally right on the other side of you at the same height as your butt? Your dick.
So, people who did more compound movements, probably ones that involve a butt flex, when going through and just after puberty, had more blood near their dick more consistently to cause it to react to the growth stuff that for men doesn't entirely stop until around age 20-22. Therefore, the people with better formed butts (which are hereditarily linked but also can be overcome. Just like the hereditary piece of the dick size indicators/factors.
