For my Husband and I, this is how we see it from a RCI perspective. Which other cruise lines it might the exact opposite
Smaller Ships such as Grandeur, Serenade, etc
Service seems more personal. Crew to Passenger ratio is closer. Seems to be more Intimate. Cruise is perceived as slower because of the more chill/relaxing time. Getting off and on the ship seems faster and smoother.
Less "Shows", not as many shops/resturants, less "entertainments". Can seem more crowded
Larger Ships such as Harmony, Allure, Anthem, etc
More things available. Shops, shows, resturants, bars, entertainment. Can seem (except when at shows) less crowded
Getting on the ship/off can seem confusing, crowded, slow ...
getting into shows or out can be overwhelming
ELEVATORS - People can seem more rude and impatience, probably due to there are more people versus the smaller ships.
For us there seems to be more children on the larger ships versus the smaller, now for parents this would be a Pro versus a Con. For us .. .our children are older and married, so not yet ready for children LOL.
So the Long Short
What is your Goal or Purpose?
if you are wanting to get away and read a book, relax under the sun, drink a drink or 5 and not be concerned what the Cruise Compass has or if you just want to Socialize ... I would recommend the smaller ships
If you're in need of constant input ... broadways, musicals, Karaoke, music halls, etc
then the mega ships are for you.
Hope this provides a perspective from a logical and functional view