Someone just wrote something that made me grasp the dynamic here.  Most people in the PCA and other Reformed denominations live out their lives hearing sermons on about ten percent of the Bible.  Galatians, Romans, forays into the other Gospels, some other Epistles and then back to Romans.

So the material for preaching has effectively shrunk.  This creates a market for more material. So when someone writes a commentary on Romans that promises to be really really long, that is a real help.  Having shrunk the Bible, a longer one is then created to fill the need.

A thick Romans commentary helps fill the Bible shaped void in a Christian’s heart.