Yeah, generally speaking it needs to be roughly 1" in diameter to fit inside a weaver mount. I'd imagine (but don't know) you could get smaller mounts, potentially. But really, you can just wrap your smaller light in electrical tape to up the diameter of it to fit snugly in a 1" mount.
The Foursevens Maelstrom is made for 1" mounts, fyi.
Anything that built into a mount already tends to be expensive and often not as bright as some of the stuff I'm suggesting. Though your lightest option would probably be a pistol light:
http://www.surefire.com/illumination...aponlight.html
But dang! That's $300!!! And actually, it's 4 oz, where the Foursevens is 1.8 oz. The surefire is 500 lumens max, so the Foursevens beats that pretty handily.
My god! I just checked out the rest of their (Surefire) weapon light lineup. They are so expensive they are not worth mentioning. Like, more expensive than the gun itself.