Lets go over the circulation of of the heart and the body. The Superior and Inferior Vena Cava, transport oxygenated blood to the heart, going into the right Atrium. This blood is then pumped into the Right ventricle through the Tricuspid Valve. The Right ventricle then pumps that blood through the Pulmonary Valve into the pulmonary arteries where it travels to the lungs and become oxygenated. Pulmonary veins transport the blood to the left atrium, which pushes blood into the Left Ventricle through the Bicuspid Valve. From there the blood is pumped from into the systemic (entire body) circulation, via the Aortic Valve.
Now think about it, the Ventricles are the main pumping mechanisms of the heart, and it would make sense that the size of the heart in certain areas would depend on the amount of work it would need to do (seeing as how it is a muscle). Pumping blood to the lungs from the heart, is such a short distance that not much pressure is required, which means that the right ventricle should not be that thick. The Left Ventricle on the other hand, has much more vasculature (and entire bodies worth), that it needs to pump to, so it needs maximum strength and pressure, which means that it's walls are going to be much bigger and stronger.
