Most of the AI-controlled factions (minus Italy & the Byz) build few boats or trading posts, limiting their economy & expansion flexibility.

My experience has been that most of the Catholic factions' expansion comes about from Crusades; then after they get conquered, they'll re-emerge in one of their previously-held crusade provinces - leading to the Bizarro world of HRE respawning in Edessa, Poland springing up in Egypt, etc.

All that said, I agree with GodWillsIt: tho' they're huge in the beginning, I've never seen HRE expand beyond Venice.