Swimming does not build bulky muscle unless it is combined with resistance training or sprints needed for muscular growth. Swimming is great exercise for cardiovascular health, stamina and building lean muscle.
Swimming fulfills the three types of mechanisms necessary for building muscle. Mechanical tension is the process of stretching or contracting a muscle. Swimming causes the muscles to be stretched passively and contracted actively during the dynamic movements that keep a swimmer moving. Maximizing mechanical tension can best be done by sprinting.
Swimming also produces metabolic stress, the process of placing the body under stress due to physical activity. It causes a lack of oxygen supply to the muscles, and it helps to improve cardiovascular health.
Muscular damage is most responsible for building larger muscles. As a muscle is exhausted, the body sends several kinds of cells to the site of the damage to rebuild the tissue, which builds the size of the muscle. Because swimming is a repetitive activity, the body grows used to the motions, and muscle damage is minimized. Routinely adding various kinds of resistance, such as paddles, fins or weights, introduces activities that build larger muscles.