The reason why Glocks hold so many rounds is because they have plenty of room to work with. There are no grip panels to fatten the grip, so the frame could be made to maximum possible dimensions to accomodate a wide double-stack magazine. With the Beretta you also have a long, narrow taper at the top of the mag which wastes room for more ammo.
Generally, between the inefficient utilization of magazine capacity, the fat grip, the long trigger, the dumb location of the safety and the super-wide slide dimensions, the Beretta is by no means the most state-of-the-art handgun design out there. Its saving grace is the design's incredible functional reliability, although the QC lapses in some of Beretta's recent production runs may have compromised that as well. Basically, as long as you get a good one the 92fs is a great gun to keep for home or shop defense, where reliability and safety features are much more important than size or efficiency.