I don't know if its just me but I noticed that in MP, the graphics are better than in SP Campaign.

You have lost your mind sir. In no way is the MP graphics better than the SP. If the MP graphics were even close to the SP they would have to have dedicated servers and im not sure if it would have run properly, all games MP graphics are dumbed down when its peer hosting, if not it would blow up your system, unless of course it was a PC.