The ventricle walls are thicker than the atrium walls since ventricles are structures responsible for the pumping of the blood to the lungs or tissues. The muscular work of the ventricles is harder and their muscle fibers develop more. The left ventricle is more muscular than the right ventricle because pumping  blood to the lungs (the right ventricle task) is easier (needs less pressure) than pumping blood to the other tissues of the body (the left ventricle task).