Hypertrophy : Increase in the size of muscles is called hypertrophy. Increase in the size of muscle may occur due to exercise. Due to repeated and forceful contraction of muscles, mass of muscle increases and so does the number of mitochondria, filaments in sarcomeres and amount of sarcoplasm.
Atrophy : Decrease in the size of muscle is called atrophy. Atrophy is due to immobilization of muscles. Mass of muscles decreases due to decrease in number of mitochondria, filaments in sarcomeres and amount of sarcoplasm. An injury to a nerve which supplies a muscle can also cause atrophy