Testis hang in scrotum with connecting vessels and muscles which contract during cold weather, and relax during heat taking testis away from body. This is a natural phenomenon and should not be bothered about. Heavy work out, however, may make the scrotum bulky by causing thickening of veins (varicocele) or herniation of abdominal contents due to raised intra-abdominal pressure. You must see a physician who would examine you to differentiate between various conditions.