Common sense tells us a pound of muscle and a pound of fat have to weigh the same, but they do differ in density. This means if you look at five pounds of muscle and five pounds of fat side by side, the fat takes up more volume, or space, than the muscle.That’s important when you’re on a diet and part of your goal is the lean look of muscle, not the flabby look of fat.his means if you look at five pounds of muscle and five pounds of fat side by side, the fat takes up more volume, or space, than the muscle.