TJ...
Here is how it was explained to me by a fellow TDR member when I asked what the differences are. His truck when you see it always has a different size stack top on it and I was befuddled why he did it all the time,besides screwing with other members,LOL.
The larger diameters change the tone from a sharper,less bellowing tone of a 4" to the much deeper and bellowing 8" & 12" stacks. Another thing he told me was how the smoke rolls out differently. Smaller 4" stacks will shoot the smoke higher in the air and the larger you go the less height you will get.
After telling me that I decided he had way too much time on his hands if he was researching smoke flight patterns with his Cummins,LOL..... Andy