Tonnage is easier to look up than square footage.

Tonnage is a measure of volume (not simply displacement) so the high overheads on QM2 Deck 2 and 3 will add more volume (tonnage) although they won't add more square footage, but I think the height of those decks helps add to the feeling of spaciousness which I think is what you're after.
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Queen Mary 2: 8 cruises
Queen Victoria: 2 cruises

ms Amsterdam: 1 cruise
ms Eurodam: 1 cruise
Caribbean Princess: 1 cruise
Carnival Pride: 1 cruise