So the revolver is a bigger ball made smaller when pushed through the revolver barrel, but would not malleting the smaller round ball in a muzzleloading rifle, when coupled with a very tight patch, make it bigger? Just a tiny bit, but a few more thousandths of an inch of rifling contact might be what makes the difference?
You are not wrong. But, I believe, there not enough resistance to expand the ball, it will just slide down the bore. Unless it is grossly oversized. I often used a .457 in a .45 Douglas barrel and the balls did distort on loading to give a very tight, and consistent, accurate fit.