[AGoetz]

I suspect it has something to do with Propulsion/Fission/Fusion/Antimatter/Gravimetric tech levels, but as I know very little about the AI routines I don't know how to fix it.

Haven't seen the AI build an armed hull larger Heavy Destroyer lately, most of the fleets are Corvette/Frigate hulls (but there certainly are some large fleets). I tend to put enough defense on a Jump Point to handle what the AI usually sends through, but every now and then a monster fleet comes through and my ships simply cannot kill them fast enough (case in point - playing as Narn, 2 Destroyers, 1 Sml Repair and 1 Cruiser sitting on each Jump point, blowing up Minbari ships. Over the space of 6 turns, 4 monster fleets (20+ ships each) came through 3 of those points. I've lost 4 of my defense fleets so far. I'm currently vectoring more reinforcements into the threatened sectors and building BattleCruisers at every available Ship Yard.