From a friend who studied "classics" as a degree, the Greeks thought massive schlongs were comical and bestial, as well as stole focus from the musculature and overall balance of the art.

Rome on the other hand considered itself a masculine nation, who's roads were broad and straight, never turning for nature.

Not that the Greeks couldn't have fun, they used statues with erect penises as road signs in Athens, famously knocked off by Alcibiades (in fact he was framed by a rival ) the night before the invasion of Sicily.

The David is a statue of a 14 year old, in addition to the reasons given in the article.