What you describe is not uncommon when playing on suicide against ultimate AIs which have huge bonuses including miniaturization or if the ships are receiving boosts from military galactic resources. Do you have the AIs set to random? If so check and see what the Terran Alliance is set to. Check if they have military(red) galactic resources which each would boost its ships attack and defense 40%. Also military starbases (not to be confused with military galactic resources) boost ships within their zone of influence.

Rather than taking on the AIs superior ships, attack its economy. Take out its starbases which are poorly defended and capture its planets (you only need to destroy the planetary defenders not his fleets). Take out its transports trying to take back the planets; if they are protected by strong fleets you can't combat then destroy the colony before the AI reclaims it. With high soldering you may be able to hold it against one or more invasions.