How does a chimney work?
In general, air from outside your home tries to enter your house through the lower portions and escape through the upper portions. A chimney system in effect draws air from the cracks and crevices all over your house (mainly around doors or windows), uses it for combustion, then vents the heated air to the outside along with smoke, vapor, CO2 and carbon monoxide.
Draft and flow are the two important concepts to understand when talking about your chimney's operation. Draft is the degree at which air is being pulled into your firebox and up the chimney. The higher the difference in indoor and outdoor pressure, the higher the draft. High temperature fires also increase the draft. Flow is the volume of air which flowing through the system. This is directly related to the size of your flue. At the same draft rate, an 8 inch round flue will always have a higher flow rate than a 6 inch round flue.