As we hinted to above, the majority of your headlamp’s size and bulk is going to come from the battery supply. If you want to keep things lightweight, look for something that uses an internal, rechargeable li-ion battery like the NITECORE NU25 or a removable 16340 like the Fenix HM50R.
The downside to these smaller batteries, you guessed it, is their runtime. Although you can easily get several hours of use out of these headlamps on a single charge if you’re not running them on Turbo the entire time, we understand you might need a longer running option.
For the longest runtimes, look at headlamps using a removable 18650 (or 2) to power the light. Not only will get a longer runtime and brighter output with these larger capacity batteries, but swapping out tired batteries for a fresh pair can keep you going for hours on end.