This has to do with blood pressure, blood leaving the right ventricle only has to travel through the pulmonary vessels and the lungs before returning to the heart.Therefore the r vent, does not pump it very hard, this creates a lower pressure, therefore the pulmonary vessels do not need to be as thick. However, the blood entering the aorta vent needs to travel a great distance throughout the body), it is pumped very hard in order for the blood to circulate. Thus the walls of the aorta need to be thicker in order to withstand the higher pressure.