A kilogram of muscle weighs the same as a kilogram of fat: a kilogram is a kilogram! Muscle, though, is denser than fat, Berkow said. "It has more water, protein, carbohydrates, and fats in it than fat does, but it takes up less space. Since density equals weight divided by volume, the same amount or weight of muscle takes up less space than fat." That's why gaining muscle is often associated with losing inches but not necessarily total scale weight, she explained (which we'll get to more later).