Son of John said:
Perhaps the time will come when society practices "selective breeding", and allow only men with large cocks to be fathers, so that all young men will have big ones too.
I understand where you are coming from, but a lot of the science research says that much of secondary sexual characteristics, including penis development is not only a result of the raw genetic profile of both parents, but a result of hormonal levels in the mothers womb. The theories being that pregnant mothers with higher androgens (testosterone) tend to predispose the developing fetus to relative larger organs (penis, clitoris), than those with more modest levels.
It is not an absolute, but there is the combined factor of the genetic potential, with the hormonal "key" to unlock that potential. It starts there.
It may even be a result of phases in the mothers life, as hormonal levels may change due to environmental factors, exercise and general health levels, etc. It may not be that healthier = bigger penised offspring, it may be more sedentary, more sexually active, or even more repeated sexual stimulation or EVEN lots of sex without orgasm that seems to leave hormonal levels high without the concomitant post orgasmic drop off.
Much more complicated than 'dad has a big one, so son will.'
I of course didn't get that boost, and my wife reminds me of the reason he haven't had sex in 6 years often. "I just never felt anything, you are so small. But oh my god, Brian touches all the spots, he is so thick, it fills me!"