The amount of force a muscle generates is proportional to the amount of muscle growth you’ll be able to create.
Force is defined as mass (the weight you use) multiplied by acceleration (the speed at which you push a weight against resistance). To generate more force, progressively increase your poundages while lifting explosively -in this context, you actually increase speed during the second half of the rep.
When you lift heavy a message is given to the brain to get stronger to face the increasing load [force]. This leads to increased muscle mass. If the load is less and the bigger muscles will have to exert less and the subsequently the size of muscles will reduce. The bigger the load the muscle will explode more and slowly the muscle mass will increase. To maintain the muscle mass or increase one has to keep exerting the muscles with higher force rather gradually increasing load.
Light training with higher repetition will lead to losing muscle mass and heavy training with fewer explosive fast repetitions will lead to bigger mass.