Muscle is the part of the body that is attached to the bones and are responsible mostly for body's locomotion. There are three types of which: (1) skeletal, (2) cardiac, (3) smooth.
This part of the body is compact and thus has a greater density because it occupies larger mass for a given volume.
density = mass / volume
Because it has great density then, the mass is also large. Thus, this accounts for a large portion of the body mass.