It's not directly related to numbers of teeth. It's the size or diameter of the cog. A force of 10 lbs on a cog with a radius of 1ft gives you 10 ftlbs. But a force of 10 lbs on a cog of radius 2 ft gives you 20 ftlbs, i.e. twice as much torque. In a similar fashion if you have 10 lbs of force available, it will be easier to turn a cog that is 2 ft in radius rather than 1 ft in radius (so long as the resistance of the cog is the same in both cases). Combining these two effects into bicycle situation means that you can apply the same force at the pedal and get more torque at the back wheel if you put a small radius cog on the front and a large radius cog on the back.