I've tried a few more games, this time vs only aggressive AIs. I'm still getting similar results though, they just don't build the fleets they use to in Diplomacy. The last game was with version .79 of the beta.
After checking the stats screen (quitting the game at various intervals) I can see that I have anywhere from 3 to 4 times the frigates on the field as each of the AIs. This is with only upgrading to the second fleet size 45min - 75 min into the game.
At this point I assume the AI's normal aggressive behavior that they had in Diplomacy has either been severely toned down for the beta or the devs just haven't gotten to fixing it for Rebellion.
Given how much the AI cheats at the highest difficulty setting it does leave me wondering what in the world it's spending all of it's resources on. I have noticed I'm always dead last in research. Perhaps the AI is placing research at a much higher priority than it should and wasting it's resources there before it even has a fleet?