Bigger is not always better in AIC. Smaller ships have substantial attack/defence bonuses. AI ships have extra bonuses too. It is quite possible your cruisers could not simply hit AI' frigates. I see it happens to mee too
One solution is to use missiles. If it against the role-playing character of your race, use small ships as well ! Or invest heavily in scanners/ECM.
You don't use fighters. It certainly makes you vulnerable. Then use point-defence beams instead of point-defence cannons They are quite effective against fighters. Also, select "don't fire on fighters" in strategies. Normal guns are a waste against AIC fighters. Hope it helps.
I strongly advise against lowering maitenace cost. Ships have build-in maintenance reductions optimized for 100%. Lowering would destabilize game balance. Use "engeeniring section" on all your ships, research Chemistry II and build maintenence-reducing buildings !