Muscles are not heavier than fat. One kilo of muscle weighs exactly the same as one kilo of fat. A kilo is a kilo. However, since muscle is denser, it looks smaller than fat.If you asked a butcher to put a kilo of pure muscle on one scale and a kilo of pure fat on another scale, you would see that the fat looks bigger than the muscle.Another way to visualize this is to imagine a kilo of cotton and a kilo of steel. The cotton occupies a much bigger space or volume than the same weight of steel.When people say that "muscles are heavier than fat", they actually mean that muscles are more compact than fat.