From a general perspective, most MP portions are designed with a more strict approach to detail in order to keep performance up for a better online experience. SP doesn't matter as much for performance because not every microsecond counts in SP. As for Crysis 2 SP vs MP visual detail, i'm not sure myself. I'd say the level designers in both SP and MP are optimising the levels as best as they can to deliver the best performance & visuals

Also if you ever played Red Dead Redemption MP, you could see how it was not planned to be something used.
Example: Go to a high mark or point on the game, pull out a sniper rifle and look around the land below and you see LOW textures and just a base color plane with no people.
I forgot how Rockstar does it, but they use a radius around your player so that if your in Central City or some place like that, then the textures and everything will not be loaded until you start going towards that area.
Its something that Rockstar used to make the game look insanely beatiful while still retaining a great sandbox size.
Crysis 2 SP might do that same and hopefully does it right.
Choregraphed Shooters, and I wont even call it this but, like Halo sometimes have bad draw distances just to have great graphics all around.
Even Comfy pointed out, the game lags alot behind a bad framerate during cutscenes and big portions of gameplay.
HOPEFULLY they did it right. But I think Crytek are all around better at making a engine that fits its games then Bungie's half assed Reach one.