No, you introduced the word “heavier”. Their comment is correct. It has about 4.5times the volume, but not 4.5 times the weight. Weight is displacement, gross tonnage is volume.
Again, you are I assume confusing displacement tonnage and gross tonnage. The Nimitz class aircraft carriers are about 100,000 tonnes displacement, and so is the “Oasis of the Seas”. That means they displace about the same weight of water, ie that’s how much the ship weighs.
For cargo vessels, displacement isn’t so significant: what is significant is volume and the weight of cargo that can be carried (volume if it’s something light like wheat, weight if it’s say iron ore). So the figures given for cargo vessels tend to be registered tonnage (gross or net) which is a measure of volume) or deadweight tonnage (mass of cargo that can be carried).
For cruise vessels, displacement again isn’t significant: what is significant is volume of hotel area . So the figure given tends to be registered tonnage (gross or net).
It’s hard to get GRT/NRT or DWT for naval fighting vessels because neither the volume of cargo or hotel space, nor the mass of cargo they can carry, is relevant.
The Oasis of the Seas has a very large GRT because it is a gigantic, largely hollow, box. I shouldn’t think a DWT figure would be readily available for her, but it’s not relevant since she wouldn’t carry much cargo. She would have a reasonably small DWT figure compared to a cargo vessel that was of similar GRT, because the latter can go very deep draft, effectively sinking most of its bulk (this doesn’t work out so well with cruise vessels, especially if you leave your balcony door open).
Very large aircraft carriers may be heavy due to all the aircraft, armour, runway and all that (ie high displacement), but would have low GRT and low DWT because they have fine hull forms that are designed for speed, and they are not gigantic floating boxes.
