Larger the ship, the better. I get more exercise, the shows are better (cruise company can spend more to hire show people with larger base of passengers), more variety of dining, and ships move with less rocking.
The largest ship I have sailed on was Oasis of the Seas (RCCL). It had a real Oasis on board with hundreds of trees and bushes and birds were living there permanently. Funny thing is it was the least crowded cruise even though there were 6000 passengers aboard.
That's a choice, for which you pay a price in the context of fewer ports of call. Which is fine. I've never been on a ship of over 3,000 and usually a bit smaller. Largely because of the itineries. I suspect your opinion is the more popular given the number of 4,000 to 6,000 passenger ships in the works. I assume there cost efficiencies, but that wouldn't be enough if they couldn't fill them.