There are two elements at work - anatomic correctness and artistic aesthetics. The renaissance was a time when art became far more figurative- people started looking more anatomically like people; there's more concentration on muscle definition and how we fit together. But, to sling a large wang in there would destroy the aesthetics of the piece and would have been considered crass to the contemporary viewer. So, basically, the tastes of the time would see a smaller penis as more genteel than a great big one.