Axiom Lazer 200. Commuted with one for 1.5 years and used it on a few tours. Works well, very small and light, and if you know you'll be riding all night you can use the light while its charging via an external battery like you use with phones. It goes up to 200 lumens and has flashing modes. Not the brightest but you said you wanted something small. It was enough to get me through some night time descents on those tours, and ride from sunset to midnight a few times.

They're about 30$ from Performance bike. I've since upgraded to a brighter Cygolite Metro 750 lumen light but the Axiom I still use as a back-up / flashing strobe light. Take alook at both models...the Cygolite is relatively small for its output, and the battery will last about twice as long as the Axiom's if you're running each on 200 lumens.

I've used both lights in complete downpours, even while charging from external batteries. No issues at all with the waterproofing.