The hotter the filament the more efficient the lamp. The upper limit to filament temperature is the boiling point of tungsten. This allows a higher filament temperature, greater visible light output and increased efficiency. Miniature high pressure gas filled lamps have much shorter thicker filaments than vacuum lamps. Short, thick wires are much stronger than the long thin filaments typical of vacuum lamps.
Incandescent lamps are quite sensitive to voltage.
Nightsun's Tri-Light and SunSport both use overdriven lamps. The reduction in light output is much greater than the reduction in power consumption. Extended operation at low voltages can inhibit the halide cycle and result in premature bulb darkening. In an effort to offer multiple light output levels from a single filament some systems employ dimmers. These devices vary the voltage supplied to the bulb.