It doesn't...A dead body feels heavier to carry, much like a person who is unconscious or drunk. The perception iof weight is probably due to the fact that a dead person does not cooperate in distributing his weight on to different parts of your body - for example, by putting an arm around your shoulder or clasping his legs around your middle. After a while, the remains of a dead body must weigh much less because much of the body degrades through the action of bacteria. (Source: The Tribune House)