What? I'm pretty sure if Bob is carrying Betty (barely) and she suddenly dies, she won't gain twenty pounds and suddenly be too much to carry. Sure, they MIGHT be easier to get off the ground on occasion but I can't possibly imagine that death adds mass out of nowhere.
And yes, corpses do weigh different amounts. This is because characters weigh different amounts. As far as I know, a character's weight is used to determine their corpses weight. So there is variation.
I think that it is important that if you can carry a person you must also be able to carry their corpse as this is one of the few ways a PC can show or and gauge each other's strength ICly.