The volume of air moving through the muffler is the same for a certain throttle position, given by the amount of combustion. i.e. it takes a fixed amount of air to combust a given amount of fuel. The force required to move the air through the muffler is directly proportional to the pressure drop across the muffler.

An extended muffler will increase the friction loss from the muffler sides, but there should be no increase in the volume of air that the engine is "pushing".

Try this experiment: blow air through a straw, feel how much resistance is there. Attach another straw on the end, it shouldn't be much harder at all to blow through both straws. Now partially block the end with your finger and blow. It should now be more difficult.