Yes, Shawn is correct. In theory, zero backpressure is optimal since there is absolutely no resistance on the exhaust leaving the cylinder, but in practice, you lose all your low-end power.
What maintains low-end torque, on a small engine is maintaining exhaust speed. There is a period in the piston stroke in which the intake and exhaust valves are both open. As the exhaust exits, it creates a low pressure vacuum around the valve, which helps draw more air/fuel into the cylinder for the next detonation. Too large an exhaust eliminates this vacuum, because the exhaust isn't moving fast enough, and your low-end power goes to hell.
[This message has been edited by gavin (edited April 27, 2001).]