One of the most common misunderstandings I here from customers is that muscle is heavier then fat. Even after thirteen years working in the fitness industry it still amazes me that people still think this. If you really think about it 20kg is 20kg. It doesn’t matter if the 20kg is made of fat or if it is made of muscle it is still 20kg. If you imagine that the 20kg of fat is a sack of feathers and the 20kg of muscle is a sack of bricks, the sack of feathers would be larger. The bricks will be denser then the feathers, meaning that fewer bricks would be required to make the weight. It is the same with muscle and fat. Fat has a greater surface area so if you put two 85kg people next to each other, the individual who had a higher proportion of bodyfat would look heavier then the individual who weighed 85kg with a leaner, more muscular body.
Muscle mass is also capable of storing water, glycogen and many other nutrients. A muscular body has a higher metabolic rate, meaning it will burn more kilocalories a day then a less muscular body of the same weight and allows the body to perform at a higher level in sports. It also gives the body an athletic appearance.