30L is 8 gallons. The average torso is somewhat longer than a five gallon bucket, but of course it's much slimmer than a five gallon bucket. An eight gallon bucket, if it existed, would be larger yet. I would question both the assumption that a torso is half the volume of a human and the further assumption that the density of humans is uniform. Obviously the thorax, which contains the lungs, is less dense than the legs, which just have muscles and massive bones with a thin layer of fat.

Sorry it took a while to respond; I got rate-limited this morning.