Metal Halides and Ballasts - Technical Question

[Saltcreep]

I can work my way around most things electrical, but ballasted systems baffle me. What determines the wattage of the bulb that the ballast can fire? In an incandescent system, a constant voltage of 110 - 120VAC is delivered to the lamp, or lamps. The total wattage of the lamps, along with the circuit voltage, determines how many amps will be drawn from the circuit. You can use any bulb from15W up to 200W, or whatever, and they all work the same.

With ballasted systems we seem more concerned with the wattage of the bulb. Presumably, it must match that of the ballast. So, how does this work? Does the ballast deliver a constant voltage? If, so, why are we restricted to a bulb that matches the wattage of the ballast? Or, are we? Can I use a 175W MH with a 250W ballast? Probably not, but why not?

This inquiring mind needs to know.