I caught a 756 lb Warsaw grouper that looked about that same size. Why would an over-grown flounder like they had weigh so much less.
Because the average grouper that length would have a lot more volume than a halibut that length.
Same with tuna or other fish. Halibut just don’t get all that thick, even at huge sizes.
Because only one side is fleshy. The other side is kind of thin.