The two dimensional size of the deck doesn't determine the displacement, it's the three dimensional volume of the hull determines the displacement. That's why some of the cruise ships are no longer or wider than a Nimitz class carrier, but they can have three times the displacement.
Both Chinese 075 and American Wasps and America are designed to transport as much as possible in their hulls. That's why they all look like bathtubs with almost vertical sides and rear. Whereas, Izumo is basically an aircraft carrier that's been called something else for the sake of law.
Which cruise ship has three times the displacement of a Nimitz class carrier?
You are confusing gross tonnage with displacement.
Displacement is the weight of the ship and defines the volume of hull below water. Gross Tonnage is the total usable volume of the ship and has nothing to do with weight of the ship. Tonnage is irrelevant to warships and is not typically calculated for warships. So warships sizes are given in displacement.
Displacement is not that important to cruise ships because thing like taxes, duties and Usable spaces are better measured with gross tonnage. So their sizes are usually given in tonnages, and their displacement figures are often hard to find.
The largest cruise ships have tonnage in the ball park of 250,000 tons because the stand so high and have so much light weight but voluminous berthing and recreational compartments above water. But their displacements, or total weight, are actually around 100,000 tons. Similar to Nimitz class carrier.
