think of it this way.
imagen your rear sprocket and crank sprocket are the same size.
1 rotation of the crank would equal 1 rotation of the rear sprocket.
now if you doubled your crank sprocket in size, 1/2 a rotation of the crank would equal one rotation of the rear sprocket.
it would be harder to pedal, but if you can rotate the crank at the same speed, your bike should be moving twice as fast/far.
make sense?