When I started shooting lo these many years ago, I got my hands on a small day horn that measured 7" on the bottom curve. I was shooting a T/C Renegade in .54 caliber and I don't recall ever running out of powder at a shooting match, but I might have filled the horn during the day. for some reason, I had been using various charges for different ranges, I think 25 grains for 25 yards, 50 grains for 50 yards and maybe 75 grains for anything further than that.
Now, I am using a nice horn that is 11 1/2" on the curve and I shoot a charge of 85 grains for everything. It has adequate powder for any of my endeavors.
Like kDub says, he only carries what he needs. After all, you're just going out to punch holes through paper or knock over a piece of steel. You're not going to go fight a war.
For hunting, I had a .72 caliber rifle that shot a 200 grain charge. I would load the barrel when I started and I carried 3 speed loaders with me. I also carried a small 750 grain flask along with 3 extra round ball and wads and patches to reload the quick loaders. Way more than I would ever need for three expeditions.