I'm wondered about this before and didn't get a good answer. If you take all the weapons and radars together, they only amount to a tiny amount of the ship's volume. What in the rest of that massive bulk, that's so important they have to build a much bigger ship just to hold few more missiles?
Well, in addition to the weapons systems themselves...let's see, there's the space for the crew, the fuel, the space and electroinbcs for the command and control, communications, radars, sonars, the ammunition, the ballast, the engines, shafts, gears, and all the piping, conduit, ventilation, etc. to handle the above...and probably 100s of other things necessary to make a modern ship like this operate.