Many cruise ships are larger than aircraft carriers! (The Nimitz class carriers, the largest warships ever constructed, displace 90,000 tons of water -- about the same as RCCL's Vision class ships.)
I feel that the Vision class ships are just about the right size. Any larger, and I feel lost aboard the ship, and there is just too much to see and do, especially on a shorter cruise. Plus, the larger ships aren't able to go certain places, the Panama Canal for example, due to their size.
From a business perspective, bigger is better due to economies of scale. From a passenger's perspective (mine at least!) -- they're big enough already!