This is interesting, and brings back some memories. I did not know the #12 caps fit any sort of revolver. These are indeed larger than #11, and up until the 1970's, as far as I know, #12 caps were pretty much "standard" for sporting arms. Those and musket caps were sometimes all you could get. My dad had a bunch of extra nipples, strung like beads on little wire coils, with a tag showing the thread size. These all took #12 caps. I still have the carton and 5 or 6 tins of Alcan #12's from a box of 1,000 (ten tins) that my dad and I bought in about 1968.
My brother has most of our father's gun tools and parts, and the last time I was over at his place, we went through some of the collection and found a trove of percussion caps. I don't remember the sizes, but was enthralled by the labels. Eley, Hicks, and others that are now long gone. I don't specifically remember any #9.
I don't know for sure, but I always thought the #12 caps were a little hotter than the smaller sizes due to a larger amount of priming compound. That may not necessarily be true, as there are a lot of other variables, but I always believed it.
Cap makers in the ammunition industry ought to resurrect the #12, in my opinion. The shooting world gets rocked every now and then by some new cartridge. How many cartridge niches remain to be filled? I wish they would pay more attention to muzzleloaders.
Best regards,
Notchy Bob