I think you will find your answer here: https://www.reddit.com/r/AskHistorians/comments/268j4q/why_do_grecoroman_sculptures_of_men_have_such/

u/Ragleur shows that large penises were associated with sexual voraciousness and even considered comical and unmanly in Greco-Roman culture. The god of fertility, Priapus, had a perpetual, oversized erection and he was often a comic figure in obscene literature. Smaller penises represented masculine self-control, large ones were ridiculed.