What do you suppose occupies the majority of her interior? Hangars and weapons stores? Barracks?
All of the above. If and when the ship is ever fitted out for a real mission, that is. Something else to consider is that the Section 31 base under London
doesn't only have those new long-range torpedoes lying around, it also has those funny looking shelves that contain row upon row upon row of sleek-looking transport craft carrying both packs of torpedoes and some much larger objects that might be phaser cannons or missiles or something.
We're not really sure what it means for Vengeance to BE a pure warship since Starfleet almost never builds those kinds of ships. Heavy firepower is a must, obviously, but as I've said in many threads, it seems that most wars are actually fought on the ground anyway and a ship the size of Vengeance would make a very effective staging area and orbital support system for a battalion of troops.