You obviously havent played my favorite map: Gateway, its the perfect turtle map.
To your issue, the AI will send the size fleet necessary to take the planet based on the defenses reported by the scout they send there to investigate.
Try this: Play Gateway 5v1, capture Gateway, and put your entire fleet there with a starbase, turretts, fighter bays, mines, planetary shields, etc., and then.....wait.
Sometimes they wont come, they'll just keep sending scouts hoping you moved your fleet. So then just build a superweapon in the second star system and set it to auto-fire. Since they can only get to the second star system by going thru Gateway, they are forced to attack.
Now, sit back and watch with amazement as 15+ caps, 200+ fighter/bombers, and 100's of other ships flood Gateway.
I tell you, when I play Gateway as TEC, and they flood me with all those ships, then I evacuate my fleet, and wait until all those ships move within range of my starbase, then hit the big red button and then *boom*...all gone.
I'll add my voice to those clamoring for more intelligent AI.
One thing that would (I think) be simple to fix is the average size of AI fleets. Even playing against one or two Unfair AI enemies in entrenchment, the AI seems simply incapable of massing a fleet with sufficient strength to punch out one of my strong points.
