David,
I think you are close in your thinking. I am not real knowledgeable either but from my old Navy days (don't want to date myself to much) the bigger the Rifle the larger the powder form. I would guess part of it is due to the composition of it. Going from the little 3 incher you might find on axualliary vessels to the 5 inchers on say destroyers to the 8 inchers on Cruisers to the big 16's on the old battleships the larger the gun the bigger the size of the powder. The three incher powder is about the size of a number two pencil lead in diameter and maybe 1/4" long. Then the 16 is about the size of a piece of chalkboard chalk and maybe 1 1/2 " long. The little one burns faster and makes a real POOP. The 16 burns slow and is more like a WHOOP sound. Fc