The following post is rated PG-13. The question arose earlier in the week during a class I TA for of why ancient men would not have showcased themselves with larger genitalia (the specific instance was when looking at casts of body parts taken to the temple of Ascelpius, the god of healing).
I knew I had read something along the way about how being well-endowed was actually a negative trait in ancient Greece. I tracked down the passage in Aristophanes' Clouds:
This is the right way for you, my lad, and if you do what I say you'll be eternally blessed with a strapping body, a gleaming complexion, huge shoulders, a tiny little tongue, big buttocks, and a small cock. Should you choose to follow the fashion currently in vogue amongst the young men of this city, then it'll be pasty skin, round shoulders, concave chest, an enormous tongue, no arse, a great hunk of meat, and a very long . . . turn of phrase!
Not much has changed since the ideal Greek body, but some aspects have. I won't get into the theories as to why this was preferred of Greek men, but most of you can probably guess.