It probably is functionally lighter, too. When you walk, your body doesn't slide smoothly along the ground - it wobbles up and down and back and forth. The way most people walk is optimised so that the torso does the least amount of wobbling, but if you add a heavy backpack on that person, it will be moving back-and-forth. Technically, that backpack isn't heavier than it it was in a person's belly, but it does move around more and cost more energy.