It's worth mentioning if has not already: Wheel/tire size matters. For the same braking rate, a 20"/406-40 tire will need to generate the same braking *thrust* at the ground contact patch as a 700c/622-40, however the smaller wheel doesn't generate/need as much brake torque to do so, and thus doesn't require as large of a disc and caliper power. Conversely, big monstrous 29er tires require even more torque. Thankfully, big wheels provide plenty of space for large discs. However, also note that the smaller wheels will rotate more for the same distance travelled, so if the same disc size as bigger wheels, the disc motion at the pads will be greater. Which makes sense, because there is no free lunch regarding energy; A given decelleration rate yields the same energy required, so less force is balanced by larger displacement.