User blog comment:Damac1214/Treyarch hints at Black Ops II Multiplayer trailer/@comment-4312627-20120727070311/@comment-92.84.110.237-20120728151342

The thing here is, all those game you listed could very well be cut in half in terms of gameplay time and still be the size of the normal black ops SP (like 4 hours, without cutscenes). And as a matter of fact, co-op did affect Halo 3 and WaW, as both could be complete way faster. Reach is different as the difficulty scales according to the player numbers.