Most of the games I have played, the AI comes at me with mostly 60+ light frigates, and light carriers. And usually only one Capital Ship. When they get Missile Cruisers and Heavy Cruisers they will send a dozen of those each along with the light frigates. But they hardly ever mass their capital ships. But for the most part they just swarm me with light frigates. If I had a starbase and a lot of defenses setup at a planet they have commited 60 light frigates, a dozen missile cruisers, and a dozen or so torpedo cruisers. With one or two Capital Ships. This one done on a map with no orbits. So it was a choke point planet.
I had a fleet of about 8-9 Capital ships but did not have the numbers of support ships. I probably had half of the total frigates/cruisers to counter them. I also had a garrison at the planet. But just one garrison mod. They steamrolled my starbase, and defense platforms. They did take some losses on their light frigates but they had so many it didnt matter much. I got my main fleet there and took down some of their numbers of frigates.
I had four heavy factories nearby and rebuilt my lost Caps, and had all my other light and heavy factories churning out ships. Gathered them together for a counter attack. In the end I never lost the planet, but lost all structures around the planet. There fleet was completely destroyed. Then I took that fleet I built and went on an offensive and wiped them out. They sent everything they had at me on that one planet.
I also noticed the AI doesnt try diplomacy at all. And it in this game I had all three AIs against me. The other two kept on trying to invade an asteroid base of mine which kept them busy. Two of the AI opponents sent two decent sized fleets there once, and even though they did some damage, being able to concentrate fire at the jump in point with the star base and defense platforms. They got decimated pretty easily. Then they would just send a capital ship in afterwards which got destroyed in 6 seconds after jumping in.
So I think their logic depends on what you have going on with your own empire. I try to get starbases as quick as possible and start building them at my planets. The AI will start sending more torpedo cruisers to counter them. When I didnt build them, I wouldnt see a torpedo cruiser but just light frigate swarms.
And to note, I beat 3 AI players with nothing but light frigates and corvettes. Never saw an AI titan, or starbase either.