How does a dead body weigh more?

It doesn’t.

Where did you get this idea?

Dead bodies do not weigh more than live ones. They are more difficult to lift, but this is a separate matter entirely.

Is this an honest question, or a figure this mystery out question?

Because he crash landed on Neptune?

There are some dozens of muscles working in concert at any given time to keep your posture, making you a fairly easy subject for lifting. Turn those off, and you’ve got arms and legs and torsos flopping all over the place, throwing off the lifter’s balance, acting against his center of gravity, etc. Your body itself does not gain additional mass.