I always call the Miracle, Pride and Spirit the "right-size" Carnival ships because they are new like all the other 2000-era ships, but they are smaller like the Fantasy class.
Big ships are good as long as they have the onboard attractions to justify the size. But if they don't then they are just more crowded.
So, to summarize. I do think the size of Royal; caribbean ships are justified - they add plenty of onboard options to justify the size/ On Oasis they have the Aquatheater, Ice Rink, theater, Central park, royal promenade, etc.
On Carnival & Princess I dont really see how the larger size is justified because they mostlyu add staterooms but not so many onboard offerings.
But take a ship lie Queen Mary 2 and I say the size is definitely a bonus (also with Celebrity Solstice) because they use the size to add comfort and grandeur - not just more people.
Still, it is a personal choice. A bigger ship will almost always have a bigger casino and more deck space. And bigger kids facilities. For families I think bigger ships are better.