I have just purchased a p61 and have had a p38 , I only made the change because of the increased btu and I had run the p38 moderately instead of always trying to overheat and thought maybe the answer is a larger btu output and run it moderately instead . the question is how does the slightly larger body 1 to 2" in length and width produce almost double the output . is the difference in the burn pot ? I notice all other running gear is the same blowers , so how are they achieving the increase .thanks