Minimum caliber requirement here in MI is .40 (where they came up with that, I don't know.....I don't think I've ever seen a .40 cal muzzleloader). I remember reading Sam Fadala's book on muzzleloading, and he said that the .32 & .36 calibers were good for things like turkey, coyotes, and javelina, but not deer, for the reasons Adam sited (ball weight, amount of powder).
I'm just curious as to where you might have read such a statement, and if they were refering to traditional muzzleloaders or possibly inlines? I have seen deer shot with pistol bullets from inlines, and all I can say is that I think this practice should be banned....huge gaping holes that were not fatal, and a long, drawn out tracking/trailing job. Muzzleloaders are supposed to be close range weapons....stretching the distance with pistol bullets isn't helping the 'sport' any.
