Testicles change size in order to stay at the correct temperature to produce sperm, explains WebMD. Consequently, when a man's body is cold, the brain delivers a message to the scrotum that causes the testicles to shrivel up and preserve heat. The opposite is also true: The brain sends a message to the scrotum when a man's body temperature is warm, which causes the testicles to enlarge and become more floppy so that the body can release heat.
One testicle often hangs lower and is larger than the other, states WebMD. Typically, the left testicle descends lower than the right, while the right testicle is often larger than the left. The average size for a fully grown testicle is 2 to 3 inches in length and 1 inch in width.
Between the ages of 10 and 13, most boys experience testicular growth, according to WebMD. As this occurs, the skin holding the testicles darkens, droops down and grows hair. Fertility emerges between the ages of 11 and 17, which is when semen is released from the testicles during ejaculation. Once puberty is complete, the testicles produce millions of sperm every day. Semen results from sperm mixing with a milky white substance.