It's possible to make games that are either too short, or too long. Depending on how much they cost, how "replayable" they are, or how well they're paced, there's a definite science to how protracted the length of a game should be.
The Assassin's Creed series of games hasn't always been the most well-paced, as the first game sidetracked the player far too often with its errands. However poorly the pacing in the first game might have been, the sequels were a lot better in that regard—if not in every other.
Speaking in an interview with VG247, the creative director of Assassin's Creed 3 Alex Hutchinson disclosed the studio's plans to create a well paced game that was "still pushing the maximum possible scope."
"The game is huge, both in physical size, play time and in the amount of new gameplay we packed in. We stripped out a lot of mechanics and systems from previous games but I think we are still pushing the maximum possible scope," he said. "There is only so much you can juggle in your mind simultaneously, and only so much variety you can add before it starts to feel random and not cohesive."
"Also, we have a very high completion rate on the franchise, and if you make it too long you start to lose that, which would be a disaster for a game like ours where we really value the consistency and continuity of the universe."
The interview, which is set to be published next week, disclosed that the game world would be roughly 1.5x the size of Rome in Assassin's Creed Brotherhood. If so, it's understandable why they'd feel the need to axe certain systems from the game.