The way I understand it, MP can be more demanding than SP since it's less controlled. A SP designer can place 5 enemies in one area to fight against the player, they know the enemies weapons, options for the player/enemies, etc. However in MP the developer can't control if all 12 players want to grab rocket launchers, meet up in one location, and just blow each other up.
One is slightly predictable/controllable, while the other isn't. Optimizing for the former seems like it would be easier than to optimize for the latter. So things may need to be scaled back or redesigned to help minimize any spikes.
On top of everything Al said, I also read that MLAA isn't the best pick for sub-HD resolutions. MLAA works better the higher resolution your game is running at. Could be wrong, don't remember where I read it, probably here.