Why does a dead body seem to weigh more than when it was alive? I assume this is where the phrase 'dead weight' comes from. I posted this in Body & Soul too, but it's actually related to an animal and I didn't want to be seen as a psychopathic murderer!.
Can't think that humans or animals gain weight after death. However when you shift a living animal you expect it to help you a bit by standing up or moving in some way to make the task easier; even if you have to trick it or lure it into doing so. Not so with the corpse; there it's all down to you, so you shift every gram of it.
When you lift somebody who is pretending to be dead they don't relax properly and consciously or subconsciously "help" you by making the lift more comfortable for themselves, ie maintaining an element of stability in their muscles which helps keep their shape. When you lift a dead or unconscious person, the muscles are totally relaxed and the body is all "floppy" which has the effect of making the lift more difficult but not heavier