Regarding ship sizes you have to be careful when comparing ship sizes.
The tons quoted for a passenger ship are not tonnes weight.
Tons for a passenger ship are a volume measurement- supposed to originate from a wine barrel size - tun. The displacement (weight) of a 300000 ton passenger ship is about half the tonnage - say about 150000 tonnes.
Where as an oil tanker displacement of 300000 tonne would be more than 300000 tonnes since oil tankers are quoted in terms of deadweight- weight of cargo carried - hence you would have to add the weight of the ship (lightweight) to get the total weight.