As Dean said, the modern cruise ships are already larger than aircraft carriers. (Though the Nimitz class has a greater range and more firepower!)
My wife and I have only been on two cruises so far, so I can't really say what I like better.
The interesting thing is that according to a shipping/cruise news site I frequent says that on June 29th, Micky Arison from Carnival Corp revealed that they are planning to follow up on the Princess plan to build a 180,000 GT ship carrying "up to 4000 passengers". Princess would take delivery in 2006-07.
Maybe it's just a warning shot at Royal...