Will underwear affects the growth of and testicles?

my husband is military. they suggest only wearing tight underwear if you are running or doing alot of physical activities (jumping etc.).this will help prevent twisting of the testicles. other than that they suggest wearing loose fitting under wear to prevent too much heat build up.this is bad for that area for many reasons besides that fact it can cause cancer and may also prevent you from conceiving a baby .genetics usually determine the size a man will be not how tight your under wear.