Next time you run the engine hold your hand about 10-12 inches away from the exhaust outlet at a low RPM. You will feel littlepulses hitting your hand. These pulses are not exhaust gasses being forcefully ejected from the engine, they are sonic pulses from the rapid expansion of the burning fuel charge. A 2-cycle engine is by nature a very wasteful piece of machinery, On every revolution some of the fuel and air charge that is scavenged into the combustion chamber is spilled out through the exhaust port.

With these two things in mind picture a muffler (tuned pipe)that has reflecting cones inside that the sonic pulses you felt would bounce off of and head back toward the exhaust port, as the pulse reaches the exhaust port it shoves the spilled/wasted fuel and air charge back into the cylinder. This increases the density of combustible material in the combustion chamber making a bigger boom when the plug fires, bigger boom=more power. The tuned part has to do with the length of the header or more accurately the distance the reflecting cones are from the piston coupled with the volume of the pipe and how theexpansion of exhaustgases is controlled for an engine of a given size, that's why bigger engines need tohave bigger pipes. A tuned pipe can be tuned to a given engine for specific narrowRPM ranges and or specific loads or they can be tuned for very broad RPMand load ranges depending on length, reflector shape, size, distance from the piston face, and volume of the pipe. The outlet of the pipe plays a minor role but is always smaller than a side dump soup can muffler because by the time the exhaust gasses get there they have cooled and begun to shrink so a smaller opening is adequate to let the gasses out.

This isa simplified explanation, from there it starts getting very technical and its discussion could fill an entire forum on its own, search the web and you'll see what I mean.