Why is the left side of the heart bigger than the right side of the heart?

Core: The muscle of the left ventricle is significantly thicker than the muscle of the right ventricle - recall that the ventricles are the chambers which contain the blood before it leaves the heart (as opposed to the atria, which are the chambers where the blood enters the heart). The reason for this difference in muscle size is because the left ventricle needs to pump blood to the whole body, compared to the right ventricle which only pumps blood to the lungs. The larger muscle size is needed to generate the force necessary to pump the blood to the whole body. 

Extension: Why does pumping blood to the whole body require more force than pumping blood to the lungs? The vessels of the whole body (systemic circulation) provide a much higher resistance to flow than the vessels in the lungs (pulmonary circulation). Therefore the left ventricle requires more force to overcome this resistance.