The AI is not very good, so in the Infinities GC, they're always given more systems to start with than the player. I believe it's like 4-5 for each main AI faction.
If you're struggling, you should be a little more proactive, be willing to retreat from battles (early on no one has Interdictors), and be willing to lose space battles but fight ground defenses to hold your own systems. You can hop around and wreck the AI's space stations to prevent them from building forces quickly, even if you don't invade to fight ground battles.