* Higher Exhaust Gas Temperature and Velocity: Exhaust gases are significantly hotter and have a higher velocity than the intake charge. A larger exhaust valve helps to facilitate the rapid expulsion of these gases, improving scavenging (clearing out the cylinder for the next intake stroke) and reducing the back pressure on the engine. A smaller valve would restrict exhaust flow, leading to reduced power and potentially damaging overheating.
* Increased Volume of Exhaust Gas: The volume of exhaust gas is often slightly larger than the volume of the intake air due to the addition of combustion products. The larger exhaust valve helps to accommodate this increased volume.
In short, the size difference is about efficiently getting rid of the exhaust gases to maximize engine breathing and performance. A restricted exhaust flow negatively impacts engine power and efficiency far more than a slightly restricted intake flow.
