Muscle size increases when a person continually challenges the muscles to deal with higher levels of resistance or weight. This process is known as muscle hypertrophy. Muscle hypertrophy occurs when the fibers of the muscles sustain damage or injury. The body repairs damaged fibers by fusing them, which increases the mass and size of the muscles. Certain hormones, including testosterone, human growth hormone, and insulin growth factor, also play a role in muscle growth and repair.