The size principle is a theory based on force requirements and motor unit recruitment. The theory states that as an increasing force demand is placed on a muscle, the more motor units and the larger motor units will be activated. A motor unit is an alpha motor neuron and all the muscle fibers that it innervates. So in basic terms the theory states that the more weight you lift then the more muscle you will activate. For example, if you were to do a bicep curl with a weight that you could curl for 20 repetitions, you will not be using all the muscle fibers in your bicep. On the contrary, if you were do use a weight that you could only curl once 1RM then you will be stimulating all the muscle fibers because you are calling upon maximum force. As you increase the weight, the higher motor units will become activated as needed. However, the large motor units are highly fatigueable and cannot perform a lot of work at once. In order to stimulate these fibers enough to illicit muscular strength and hypertrophy gains you must allow them rest. As with all muscle fibers, time under tension, or TUT, is a huge factor in cell volumizing. It is for this reason that single set exercises to failure are not as good as some people think. If you choose a weight that you can perform 15 reps with, it is not enough to stimulate the higher end motor units and TUT for those fibers is zero.