Hmm...if this game takes place all inside Arkham, I can see two things:
1) The game is somewhat short.
2) Arkham is a HELL OF A LOT bigger than previously established in any incarnation of Batman and has a TON more prisoners (assuming that Bats will face off against many henchmen).
Not that #2 will make or break the game. It'll be just an interesting observation, if true.