One difference between the largest Celebrity ships and HAL's Pinnacle class ships is the Passenger to Space ratio--the enclosed space (measured in cubic feet) per passenger. The higher the space ratio, the more guests will have a sense of spaciousness on board. The ratio for almost all of Celebrity's ships is in the 35 to 37 range. Most of HAL's ships are in 34 to 36 range. Not a big difference there. But K-dam, Rotterdam, and NS have a ratio of 31.
I have only anecdotal evidence to suggest this number matters. Friends of ours were on the Konigsdam after several cruises on Celebrity ships. They reported that the K-dam felt crowded and they had trouble finding any public area that wasn't full of people.
If you want to know more about this subject, google passenger space ratio. The only HAL ships I've been on, Veendam and Zaandam, have the highest space ratios in the fleet--36. The Celebrity ships I've taken have a ratio of 35. I never felt crowded on either line.
The space ratio is based on a ship's capacity, so if a ship is less than full, passengers will feel it is more spacious. Your mileage may vary, as the saying goes.