Bodybuilders use weight training to improve their body image by developing their muscles for size , shape , and symmetry regardless of any increase in strength for competition in bodybuilding contests ; they train to maximize their muscular size and develop extremely low levels of body fat . Bodybuilders use weight training to develop their muscles for size , shape , and symmetry regardless of any increase in strength for competition in bodybuilding contests ; they train to maximize their muscular size and develop extremely low levels of body fat .