you cant compare ship sizes in freelancer. It's like saying our ships hold a billion people each because they're almost as big as the planets in the game.
Objects in space are (smaller or larger) than they appear, scaling is not done properly in freelancer, and for all we know the order light carrier could be twice as big as the rheinland BS.
Still, I prefer Jinx's explanation - carriers are usually designed with multiple launch bays or tubes to allow rapid deployment and retrieval of fighters. While a battleship may have more storage room for fighters, their launch bays dont allow more than one or two ships to launch at a time, and most battleships only have a single launch bay.