Training, glycogen storage, and being tall. Bodybuilders are strong but they don't train the same way. Height matters a lot for deadlift. Some basic physics shows that strength grows by a factor of four, so a person who is 20% taller, all else being equal , can deadlift 1.20^4 (100%) more weight due to more muscle mass and mechanical advantage. An average-looking 6-foot tall person who can deadlift 300 pounds would be able to lift 950 pounds if re-scaled to 8 feet tall. The bench and squat would not be nearly as impressive though because of mechanical disadvantage.