I learned this in art history myself so I'm surprised she couldn't find it in the books, but there's a double reason. In classical times, a smaller uncircumcised penis was considered a trait of nobility and desired; while being both hung like a horse and circumcised was a trait of the uncivilized outsider (the African, Arab, or Jew). 2nd, the artists responsible for such intentionally go smaller when it comes to the genitals to avoid it from being the focal point / first thing you see when looking at the work.
For all we know, David's a grower and that thing blows up by a few inches when erect (not to mention it's probably really cold in there).