In recent historical discussions around the internet, the topic of male genital size on ancient Greek statues has been notably addressed. I honestly don't see why it matters (and I will get to that at the end of this post), but as a Hellenist who has studied since 2009, I'd like to think I could put forth some valid theories. Many ideas, to my knowledge, have been proposed, all the way to the suggestion that the Greeks admired small genitals. But I think there's another facet that we are failing to see.
For starters, it's not true that all Greek statues or depictions have this attribute. The Herm piece or a depiction of Priapos (a fertility God), will show very large penises. While other statues, like the one of Zeus or Poseidon of the Artemision Bronze, do not have large ones. But I think the factor is in the area of focus. With Priapos, the genitals were a focal point because of the immense fertility the God possesses. But in the case of the Artemision Bronze, the sculptor clearly didn't want genitals to be the message of the work. What you see is either Zeus throwing the thunderbolt or Poseidon throwing the trident. The focus is on the God's Divine power, not the genitals. So my theory is that it depended on where the artist wanted the viewer to look.
I also think that many people assume that male genitals are always the same size. This is not true at all. When erect or exceptionally aroused, a man can become several times larger, depending on how much blood flow and stretching the penis can hold and exhibit. So it may have been that the sculptor wasn't seeing small genitals, but rather, simply ones that were not erect.
Finally, I find a notable amount of sexism behind the whole discussion. If we were to see a statue of a woman with small breasts, we wouldn't think anything of it because we would consider that there is more to her life and her being than just sexual, but for some reason, when we come to a man, we put all the worth and focus between his legs.
In the Goodness of the Gods,
Chris Aldridge.