USB, as in charging, or battery pack attached?
Fenix HM50R is the smallest that I have - great little light!
Hm… I dont know… Like nitecore… They have very small headlamps… With battery pack.
I mean, that depends on what you’re looking for. Streamlight Bandit is pretty small
Nitecore NU series are maybe the style your looking for.
The bulkiest part of the headlamp is often the strap. The petzl zipka has an interesting minimal retractable strap, but the rest of the light doesn’t look great.
Nitecore NU05 Kit 35