Because piston rings don't seal 100% you have to have a place to vent the blow-bye. A PCV valve's size is determined by someone's best guess as to how much venting your engine needs to maintain normal pressure. Engine size and condition will change an engine's venting requirements, too much venting and you will get moisture in the
oil, too little and you will get
oil leaks and overloaded
oil rings. PCV valves were developed because the government cares more about emissions than about the inside of your engine.
Scott