In Greek statues in particular, otherwise perfectly accurate anatomically , this is a weird convention. Painted pottery is more stylized, but also has the same micro penises. Was this considered attractive, or why was it?
First, not micropenises & question the sincerity of this post... but in benefit of the doubt here's a study about real penis length.
https://bjui-journals.onlinelibrary.wiley.com/doi/abs/10.1111/bju.13010
Second, Classical Greeks believed all that is human resides in "mind" and "reason." The body is no different than any other body of any other animal. So, the scholars who leave us the art we see believed a large penis - since we can all agree lust is not very reasonable - means a man is more animalistic than a man with an average penis. Then there's the whole taboo around the glans being seen so athletes would tie the foreskin closed when competing nude. I'll let you Google this information since it's easy to find as the West is obsessed with Classical Greek nudity & basically every philosopher of the time explained the animal body v human reason distinction.
Lastly, where do you live if you think the statues are anatomically correct? Very beautiful neighbors you must have! The statues are idealized, to better honor the gods which the sculptures represented. The ideals of how the gods looked most likely came from soldier heroes in history to the Classical Greek artists who, due to the rigors of soldiering, had similar body types. Even statues of presumably real people, like David, would have been teenaged boys or girls who often have zero body fat & muscles from the physical work & diet that would have been done by all but the richest. Since art honored the gods & humans as the closest in all creation to the gods, why sculpt a rendition of some schlub who do not exemplify the perfection of those gods & humans?