Originally Posted by WhyFi

Here - put away the cogs for a second and think about levers. Walk over to an open door, and push it at the outside edge of the door, by the knob. Easy, right? Now push it from the middle of the door, halfway between the hinge and the knob. Harder, yeah? But you don't have to push it as far to get it to swing closed. Now push it 3/4 of the way towards the hinge - even harder effort is required, but you don't have to push it more than a few inches for the outside edge of the door to travel a pretty big arc. Same thing with the rear wheel of the bike. The closer to the hub (hinge) you apply your effort (the pulling chain) the harder it is, but the farther the outside (rim/door knob) travels.