Dome size, 8 vs 10??

This should be easy but I am not thinking right now....

What difference does it make having a larger or smaller dome for a heat lamp, such as a 8.5 or a 10.5? Spread out the heat, or dissipate the heat? What circumstance should they each be used in?

I would expect that the biggest difference to be made will not be with the size of the reflector, but rather the wattage of the bulb/element that you put within in.