Originally Posted by Polaris OBark

There is an 8% difference in length for 165mm vs. 180mm, and about 4% for 165 vs 172.5 mm arms (a more reasonable comparison).

Torque = force x lever arm, so when the arm is perpendicular to the force, you get a 4% or 8% increase in torque (compared to 172.5mm cranks, and 180mm cranks, respectively). When it is parallel, it makes no difference.

For some applications, these small differences might matter, but for most people, they don't, especially when compared to issues arising from pedal strikes or range of motion.