You have a traditional problem, made extreme by the levels involved. In broad terms, consider cutting current in half. Ignoring the funny resistance vrs temperature thing with bulbs (not trivial), considering them straight resistance, this means half the voltage drop. The battery stays the same voltage, meaning that the difference, half the total battery voltage (same as lamp voltage in this case) is 'across the series resistor'. As another poster has hinted, this means not only is half the power we're taking from the battery wasted in heat, but the resistor in question must be robust indeed. Ironically, often lamps are used in similar cases since they can get rid of a lot of energy as light rather than heat directly. True, the total battery current is half the stock case, but in terms of watts, we've wasted half.
However, lamps work by heat. If we turn it on and off ('full blast' when on, zero when off) fairy quickly, it all 'blends together' as heat. This is how household light dimmers work. Even though they control hundreds of watts, they stay cool since they are nearly perfect, high speed, 'switches'. Always either full on or full off, just changing rapidly. The lamp filament, because of it's thermal mass, responds to the 'average current'.
As you might guess, such circuits are challenging. If it hangs up you have problems. Either too bright, not lit or burnt up (Murphy enters favoring the latter historically). There can also be thermal issues in the controller. While higher temps favor us usually (gains go up for instance) sometimes it works against us (leakages go up as well). Thermal run away is a common fault mode.
Then there's the whole issue of poor lamp performance at lower drive levels.
If you want less light either buy a lower powered unit, or use a lower power bulb in the one you have. If you want the 'quick and dirty' solution to variable power, try some hunky diodes in series. High power diodes are often cheaper and easier to find. It's also possible to make a switch selectable power setup with a string of them (you get .7 or so less volts per). If your current is under 6 Amps, Jameco has some attractive ones in the 'easy to deal with' T6L case (no heatsink needed) at 29 cents each. Check out their part number 177762. As an added bonus to such a system, you can switch diodes out to maintain light output as the battery discharges.
Or so I see it.
Doug Owen