I am playing on genius and have the problem that the AIs never finish each other off. Once an AI is pretty much defeated and most ships/orbital infrastructure is destroyed, they suddenly get tons of cash. Im guessing this is the bonus income they receive from the difficulty level and not spending it on any maintenance. At this point the AI wars are always ended, so I assume they use their boat loads of cash to buy peace. The problem with this is that the AI empires never grow at the same pace as me because they never actually finish off any other AIs to take their space and resources completely.
So my suggestion is to nerf peace making between AIs a bit so that I can get some big empires to face mid/late game.
I am playing 1.7 beta and loving it, small things like this make the later game a bit dull though