It has to do with how the torque is transmitted from the pedal gear and the wheel gear. The chain transmits the force equally, but the change in radius produces a change in torque.
So if the wheel and the pedal gears are similar in radius, the behavior is similar to riding a mono-cycle: all torque applied is transmitted unchanged.
The opposite case is when the wheel gear is smaller radius: the torque applied is reduced in the wheel.
Now, you may wonder: "Then what is the point of having gears that reduces the effect of the force applied?"
The thing is that both are linked by the chain, and this also links the rotational speeds of the gears, so if the gears were the same radius all the time, the faster you would go means you would have to rotate much faster, to increase your linear speed. This situation is much better when the radius of the wheel gear is smaller, since small angular rotation in the pedal gear, multiply and generate higher revolutions in the smaller one. Hence, you can make it go faster maintaining a normal pedal rotation for you.
Check out the image that illustrates $\tau_1 / r_1 = F_1 = F_2 = \tau_2 / r_2$. But always bare in mind that $\omega_1 = \omega_2$.
