I have Fenix, Titanium Innovations Illumina Ti and ITP A3 AAA lights. All are very small and put out a claimed 80 to 115 Lumens. The Fenix has the brightest center spot and longest throw while the Illumina Ti has the widest center beam with the ITP about mid way in between. All are very compact lights about 1/2" in diameter and under 3" long. Very bright output keychain size lights. I do not know of a handlebar mount designed to hold lights that small however. The ITP is the best buy as the 3 brightness version is about $20 from Battery Junction.

Take a look at the Olight I 15 Infinitum. This is an AA light that claims 140 Lumens with one AA battery or 210 Lumens with a 14500 Li Ion AA size battery. The Infinitum series has continuously adjustable light output from minimum of 6 Lumens to maximum. The I15 is about .75" to .85" in diameter and 4.25" long. It fits in a Twofish light holder fine. Very versatile as it can use anything from a 1.2V NiMH to the 3.7V 14500 battery. In fact battery versatility seems to be a feature of many of the Olight flashlights.