@MasterCheeze: While I agree that’s a general trend throughout history, I think it’s been reversed for the HD consoles this generation. With the massive development costs that come with a game for either/both of the HD twins, it’s simply impossible to make a length adventure and keep the game profitable. Even games like Halo 3 and CoD4 had incredibly short single player experiences despite the knowledge that the games would sell insanely well. With all the new gamers’ obsessing over online multiplayer (lol real life friends?), it’s easy to get away with it too. HD games that are epic in scale (i.e. FFXIII or GT5) require huge development times, and need massive sales to just break even.
Your trend still holds true for the Wii. For example, the Conduit, which has only sold a fraction of the aforementioned games features a campaign that is nearly twice as long and was profitable, too.