It is said that the ideal body type of an Ancient Greek male had a small penis. It was even comically mocked in art when men had larger penis's. What was the reason for this?
/u/papiriuscursor has previously addressed a similar question about classical statuary.
/u/Kelpie-cat has previously compared nudity in classical art to nudity in ancient south Asian art
/u/Toldinstone has previously explained why some male statues have the penis removed
EDIT: Other redditors may have more to say on this topic.
This has quite a lot to do with Greek philosophical concepts. Namely, the longstanding interest the Greeks held of Virtue. Virtue, when defined, is a sort of moral excellence that many believed (as did Aristotle and plato) was central to leading to the good life. Virtue, described by Aristotle, is held to be the "middle aspect" between to extreme. Temperance, it might be said, is the middle ground between lust and sexlessness. One ought to be temperate, because it is ethically good.
It's also important to remember that it seems that the ancient Greeks did not have such a strict separation in the roles of art and moral philosophy as we tend to hold these days. Classicist Martha Nussbaum has argued that the reason Plato is so critical of playwrights, storytellers, musicians, and artists is that he saw them as his philosophical rivals, and believed that these sorts of people misled their audiences morally. Good art, for Plato, must be in accordance with moral truth: virtue. Moral truth, for Plato, did not involve the emotions (which him and others saw as irrational), which is another reason why he mistrusted his rivals: their "arguments" were emotional.
So, now to the flaccid penis.
Plato was not the only one who held the virtues in high esteem, indeed many artists and their patrons did. So, when you see depictions of heroes, you will see them in accordance with virtue. This includes the flaccid.penis, an indication of their adherence to temperance, which indicates their moral goodness.