The myocardium has four chambers, two atria superiorly, and two ventricles inferiorly. The left ventricle of the heart has a much larger and thicker mass than any other chamber. The reason for this is because of increased pressure in that chamber. When blood leaves the left ventricle it is propelled into the aorta(largest artery in the body) into the aortic arch then to the thoracic aorta, then the abdominal aorta. From the abdominal aorta it bifurcates(splits) into the iliac arteries and then the femoral arteries in the thigh's bilaterally. The left ventricle is thickest so it can accomodate the increased pressure and volume of blood that the heart pumps. This volume is called cardiac output, about 5-7 liters/minute.
The heart is divided in four chambers, two upper chambers called atria and two lower chambers called ventricles. The thickness of wall muscles of these four chambers varies according to the pressure with which they need to pump the blood. The atria pump blood in to ventricles, which does not require much pressure. Therefore walls of atria are thin. In comparison ventricle pump blood with greater pressure and therefore their walls are thicker. Among the two ventricles, the left ventricle has to pump blood throughout the body, while right ventricle pumps blood only to lungs. As a result left ventricle has to pump blood with higher pressure as compared to right ventricle. For this reason the walls of left ventricle are thicker in comparison to walls of right ventricle.
The ventricles are chambers in the heart which collect blood from the atrium (a smaller chamber) and then pump the blood out of the heart. The walls of the ventricles are thicker than the atrium walls to allow them to hold up to the higher blood pressures in the ventricles which allow them to pump blood easily to the body and lungs.
When comparing the left and right ventricle, the left one has thicker walls than the right because it needs to pump blood to most of the body. The right ventricle just sends blood to the lungs.