To add on to Steve's note that the longer focal length inherently provides better divergence, it's also important that the diameter of the lens is large enough to capture the laser output.
A longer focal length lens must be positioned father from the light source in order to produce a parallel beam (approx. the same distance as the focal length).
The G2 is a short focal length aspheric lens which is able to achieve higher transmission than some other common collimators because it can be positioned close to the laser diode and capture all of the light. The aspheric design reduces spherical aberration, so offers additional improvements over simple plano-convex designs when it comes to beam profile and the focused spot size.
Of course the drawback is higher divergence as a function of the small beam diameter which isn't a problem for single mode laser diodes but is very apparent with high power diodes that have inherently lage divergence angles.
