Simple.
Look at all the territory the UNSC expanded into until it ran into the Covenant. If you assume that the sapient species in the galaxy are evenly distributed, then the Covenant's territory is at least seven times (Brutes, Drones, Elites, Grunts, Hunters, Jackals, Prophets) the size of the UNSC's, and I personally believe that it's much larger.

So, the Covenant would have had a much larger fleet to keep the peace in their territory. On top of that would be the amount they produced for the war effort. Take a look at this chart (from 1940-1945) for an idea of how balls-out their production might have been.