I think it has something to do with the designcreation AI file. The AI has set roles for ships and i'm sure they have a Maximum tonnage for that kind of ship, so i think if you increase ship sizes it might conflict with that file. Not sure though as i am no AI Modder
As for decent TDM AI's i'd suggest The Rage and Vaxin. The other challenging ones are the ones you've already played against.
