If you put a larger sprocket on the front and an even larger one on the rear to get the same gearing, the "torque" advantage you speak about will negate itself. It would take more torque to spin a larger front and less to spin a larger rear, think of where the power is coming from and where it's going. The overall gearing would be the same so the RPMs at any speed would be the same. You're adding mass that you have to accelerate both in the chain and the sprockets. You might get less resistance in the bend of the chain around the front sprocket, but the added weight would ofset this as well.