It is scientifically proven that where weight training is concerned, you don't have to pump massive weights to reap the same results as the guy who lifts baby weights. You just have to do more for lighter weights than working to heavier weights with lower reps.
If you think about it, it makes sense; the actions of our skeletal and muscular system are not repetitive to fatigue extents unless you are training which is why unless your grosely obese or medically unwell, you don't get out of breath performing simple daily tasks like walking around. However, you look at videos on YouTube of lads who have totally transformed themselves with Calisthenics. I'd argue their results far exceed those of any weight training program when coupled with a helathy diet and plenty of sleep. Theirs is similar in action to Yoga in that it's simple body movements using your own body weight and stretch.
I'm sure someone will correct me on this