I've seen a few of you hit upon it, but I also find that the ratio is more important than just the numbers of bowl size being larger numbers. For example... I find my Piumas at about .5" x 1" deep to smoke for about as long as m .8" x 1.7" deep pipes. The Piumas hold about half the tobacco, but the ratios of half as wide as deep all seem to smoke for about the same length of time.
I also find that my tall conical chambered pipes tend to smoke for longer than my more U-shaped chambers... I am not sure why that happens.
I also find that my tall conical chambered pipes tend to smoke for longer than my more U-shaped chambers... I am not sure why that happens.