There's an obvious answer to the smallness of statues' penises: the manners and religious prudishness of classical elites. But the issue is more about differing standards of beauty and modern mens' penis anxiety, writes Ellen Oredsson. Which is to say that smaller penises were once regarded as ideal, and many real penises aren't any bigger than the ones on the statues.
…small penises were more culturally valued is that large penises were associated with very specific characteristics: foolishness, lust and ugliness. There are actually quite a few ancient Greek sculptures that have enormous penises. Here's one:
Small dicks are, then, associated with reason and logic. The argument gets strained when applied to the western renaissance, where imitation and idealism intersect more sharply with religious sentiment.