Both the current and voltage contributes to the brightness. It just depends on the configuration. If an incandescent is 100% efficient (with it isnt btw), that means all power (V*I) would be used to produce visible light rather than 90% being wasted as heat.
"Brightness" is a measure of power, so at constant voltage power is I^2R. If resistance is decreased and current is increased by an equal factor, the increase in current counts for more and the bulb will appear brighter