How does lean mass compare to fat?
Muscle cells are tightly packed with myofibrils. When these contract with enough intensity, the body adapts by generating more myofibrils and sarcomeres (assuming proper training and nutrition), increasing the density (and strength) of the muscle.
Even denser, bone is composed of complex combinations of calcium and phosphorus, heavy minerals that provide strength, flexibility, and support for all the stress we put on them. Bones also contain a significant amount of protein (mostly collagen-type proteins).
Adipose (fat) tissue, on the other hand, is loosely composed of adipocytes, cells that contain light, fluffy lipid molecules (mainly triglycerides). Unlike bone and muscle mass, fat tissue provides unlimited storage all over the body, so it will continue to grow when we over-eat.
This means: Muscle and bone are 18 and 33 percent heavier than fat by volume. It also means that your exercise and nutrition plan can help you look (and function) better without leading to weight loss.