There's a lot to it but the basic principal is that for induction you have at most 100kPa to push the air in (atmospheric pressure). For exhaust there's typically around 400kPa above atmospheric pressure when the valve opens and this pressure is dumped/ wasted. That means to get the air moving you have roughly 4 times as much pressure available at exhaust so you can use a smaller exhaust valve to make room for a bigger intake.

There's also some fluid mechanics going on there, the fact that sudden expansions cause more pressure drop (losses) than sudden contractions, again this means you want a bigger intake valve than exhaust.

In a 16v head the exhaust valves would be bigger because there's room to fit them and get a better exhaust flow. For each engine size, valve timing and rpm there's an optimal port/ valve size.