A full tuned pipe will have a low pressure wave reach the exhaust just as the piston uncovers it. This helps extract all the exhaust plus it draws some of the fresh charge through as well. But before the exhaust port gets closed another reflected high pressure wave reverses the flow of fresh mixture and pushes it back into the cylinder. This is where the "supercharging" effect comes from. Because the waves inside the pipe travel at a constant speed the engine must be running within a certain rev range for the timing of these waves to be right. By lengthening the pipe the interval between the high and low pressure pulses is extended and so the engine needs to run at lower revs to match this extended interval. OTOH by shortening the pipe then the engine must be run faster to be "on the pipe".

A muffler works mainly by allowing time for the pressure changes to get all mixed up and dampen out before going out the end. The bigger the muffler (more volume) the more time for the pressure to even out and the quieter it'll be.