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John,
Though the exhaust gases are expanding it's the piston that is pushing them out particularly at the end of the exhaust stroke. The long duration exhaust helps to improve the volumetric efficency of the engine. That is the complete evacuation of the exhaust gases after each cycle. This leaves a clear path for a fresh charge of intake. The header plays a role here too. If properly designed the header will also help evacuate the cylinder of spent gasses.
I was also taught that the exhaust valve is smaller due the idea that there is less mass after combustion than before. If you think of the internal combustion engine as an air pump, where the intake has a portion of solid mass (gasoline) that is converted into a gas during combustion then there is some validity to the smaller valve. But it still important to evacuate that gas once converted.
The turbo motor is a comletely different case from a naturally aspirated motor because you are blowing the gases through the motor. It is not unusual to see turbo motors with a volumetric efficiency in excess 110%.This is because the turbo not only blows out all the exhaust but also some of the new intake. A good example of this is a top fuel car. At night you can see spectacular flames from the exhaust. You will also notice that turbo headers are smaller than naturally aspirated motors in order to keep the exhaust velocities high enough to spin the turbo.
Hope that helps,
neilca
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