Most games as well have the ability to limit super fleets with a population limit. Most times, they're just imposing, and I'd imaging that they would be very imposing in EAW due to the fact that you have to have a space fleet and a ground army. Superfleets also give a disadvantage in a way if you have the whole thing grouped together. In that case, unless the enemy sends in one attack from one direction, youre gonna be hard pressed to split up the big fleet and defend the other attacked areas. Where I can see having a larger fleet spread over a larger area (2 Star Destroyers with one planet, 3 iwth another etc), having one fleet isn't always advantageous just because of the limits of defence of such a large area in EAW.