Increased work load will lead to increased muscle size. Work load can be increased in multiple ways and one of the best ways is to reduce the resting time and doing your workout in a shorter period of time. The amount of work in each workout is a function of the weight and the number of repetitions over time. Just reducing the time between sets and moving on to the next muscle group quicker will result in more work done in a shorter period of time. A lot is said about increasing the difficulty with each workout, but almost never will it include advice to reduce the time in the gym.
A simple way to accomplish a progressively more difficult workout would be to keep the weight constant but to use ninety seconds of rest on the first day, then sixty seconds the next day and thirty seconds the third day. Increase the weight for the next visit to the gym, again starting with a ninety second resting period. Don’t be surprised if you struggle to complete the workout. Using this method is definitely a humbling experience. Be prepared to get out of your comfort zone to increase muscle density and take your fitness to a new level.