Contrary to popular beliefs, you can build muscles using lighter weights the same way you would using heavier weights. What heavyweight does for you is to help you improve your strength and not your body mass. Note that there is a difference between strength and muscle mass. It, therefore, suffices to say that it is not a function of how much weight you lift how well you can tire out your muscles and cause the synthesis of proteins.
In summary, lifting heavy weights is not the only way to increase your muscle size.