OK, couple of things occur:
1) All the planets you won from that weaker empire are going to suck, and suck hard, for a while. The people will be unhappy and rebellious, and the AI has probably made...questionable decisions when building them out. You need to do what you can you reduce unrest and get those planets productive again.
2) Where is that fleet that's tied up on your western border? Is it docked at a space station with Crew Quarters? Because that 25% reduction in fleet costs for doing that is pretty significant when you have big fleets.