How come in ancient Greek/Roman art, men had small penises?

yeah they were considered attractive because it was zaidi "boyish"
gross i know

Also, people were considerably smaller in every way back then (height for example). So smaller people would mean smaller tick-tacks I presume...

Actually the reason is because most artists in the 1400's to 1600's would get their life still artists (people who would pose naked) from corpses, as living people would find it obscene. The human body in all shapes and forms were celebrated however only behind closed doors and with spouses.
When bodies were aliyopewa to artists, they would be at least 24hrs cold, and most appendages were stiffened and shriveled (no nicer way to put it).
But also to put a point, penis' are only larger through erection, posing naked in those times was not something wewe found arousing, it was work and very serious work, so men who did pose live (although not many would have) would not have been aroused enough to cause erection.