One muffler shop dude told me (and it made sense) that the exhaust is hot near the headers and cools as it travels out the back. That's why you can hold your hand over the rear exhaust pipe but not at the header outlet (too hot). Cooling gas should condense.....so the same amount of exhaust gas (volume) at the header should decrease in volume by the amount that it cools (temp) off as it travels to the rear.
