There are different types of Mega ships. RCI, NCL and Carnival have the mega ships with all the latest and greatest things to do on them, Water slides, rock walls, sky diving simulators, surf simulators, bumper cars and what not. They have greater selection of entertainment but the venue's are smaller so not everyone can go. On some of the new ships you have to scheduled and make reservations for shows and activities.
Celebrity and HAL also have large ships that are not floating amusement parks (Solstice Class ships) That have lots of space per passenger so you do not feel packed in like a Sardine by the pool or going to the MDR.
Cruisingaddicts hit the point, # of passengers for the size of the ship. for example
âQuantum of the Seans 167,800 tons 4905 passengers = 34.02
Celebrity Equinox 122,000 tons 2850 passengers = 42.81
NCL Epic 153,000 tons 4200 passengers =36.43
Carnival Dream 128,000 tons 3646 passengers = 35.14
HAL Eurodam 86,000 tons 2044 passengers = 42.074
HAL Ryndam (old ship) 55,451 tons 1266 passengers = 43.80
The ship you pick will dictate the vacation you will have, high energy crowded or relaxed and spacious. I can appreciate both and have had good times on both. I will say for a short trip 7 night or less the high energy crowded ship is fine, for longer cruises I prefer the more relaxed spacious ships.