The right lung has three lobes because it does not have to make as much room for the heart. The left lung has two lobes because the heart is found just left of center in the chest.
The left lung has a cardiac impression to make room for the heart. Some scientists believe that the left lung once had a third lobe, but it was lost over time.
The right lung not only has more lobes than the left, but it's larger, heavier and has more capacity. It's separated into the superior, middle and inferior lobes. These lobes in turn are separated from each other by the upper and lower fissures. The upper and lower lobes are about the same shape and size as the lobes of the left lung. The middle lobe, which only the right lung has, is the smallest of the three.
The right lung does have an area where it accommodates the lining around the heart, or pericardium, but it's not as deep as the left lung's cardiac impression. It also has an arch that makes room for the azygos vein, which brings deoxygenated blood to the superior vena cava. There are also furrows for the superior and inferior vena cavae and the innominate vein and artery.