on a 4-stroke engine, NO backpressure is a good thing...but completely eliminating it is near impossible.
running too-large an exhaust pipe reduces exhaust gas velocity, which allows the exhaust to slow and cool inside the pipe, effectively "plugging" the pipe and INCREASING backpressure.
with a properly sized pipe, you maintain gas velocity...and as each exhaust pulse travels through the pipe, it creates an area of low pressure behind it, which helps to pull the next pulse out of the combustion chamber and down the pipe....this occurs in a continuous cycle, acting like a sort of "reverse supercharger", if you will, pulling the exhaust out of the engine. this effect is known as scavenging, and is very important for proper performance.
so, using too large of a pipe will increase backpressure by allowing the exhaust pulses to slow down and build up inside the pipe, and it also destroys proper exhaust scavenging. the double wammy is enough to really decrease performance.