You didn't say what brand the caps were or if the #11's were magnum caps. BUT---as far as any difference between standard #10's and #11's in firing (setting off a charge of powder), there is none as far as any pressure increase. There could be a slight difference between brands, but not enough to detect. Magnum caps aren't needed for setting off any revolver rounds, unless as I've read yer using some substitute black powder. The only difference between 10's and 11's is the length of the skirt. They have the same internal dimensions. The longer skirt on 10's allows them to be seated farther down on the nipple giving them a bit tighter fit. (CCI 10's though are tighter, see next paragraph).
I've been shooting cap revolvers since 1972 and have used both 10's and 11's, Remington's, CCI's, RWS, and some other long forgotten brand. I've always liked Remington 10's on my stable of Colts, Remington, and Ruger Old Army. They fit better down on the nipple and have rarely, rarely had any loosen up on recoil. The brand and size of the nipple makes alot of difference. The factory Uberti and Pietta nipples always worked best with 10's, but 11's (Remingtons) worked, CCI 11's were OK also, but I like Rems better. The after market nipples like Slix, Tesco, and Track of the Wolf I've used 10's. The only cap I found that wasn't worth anything is CCI #10's. They were to tight fitting on any of my dozen or so cap revolvers, no matter the brand of nipple, except one Pietta Remington cylinder I bought to go with the revolver. Anything else I couldn't get them to seat fully.
Give your #11's a try and see how they stay on when firing and for fit. You mentioned having a Walker. If is is a Uberti like mine, #11 Remingtons worked, but I like 10's better for the longer skirt. You won't notice any increase in pressure between sizes no matter the brand. As I mentioned at the start, magnum caps are sometimes used by shooters using substitute powders from what I've read, you won't blow yer gun up, although you could see some change in accuracy I'd think, not needed though. Standard caps of what I've mentioned set 45-55 grains of FFF Goex powder off fine in my Walker, 40-42 in Dragoons, 30 grains in Remington and Colt Army models, and even the Ruger OA charges, except CCI#10's. Have five tins of those that have been sitting doing nothing for a spell.
(Edit) Ifin ya want to read a great 'master piece' on caps and nipples/cones, if yer familiar with CAS City forum or if not, go to The CASCity.com forum, The Darksiders Den, then click on The Dark Arts. About 2/3 down on that page there is a great thread put on by a guy called Mako, (very knowledgeable) titled "Capgun Primer: Correct Cone Length". Mako gives all kinds of cad drawings of revolvers and nipple/cone dimensions, needs, and all of the most used brands and sizes of percussion caps, graphs, the whole nine yards. Check it out and you'll almost have a PhD in percussion caps and nipples. Very informative. It was written back in 2011 before Slix nipples were out and Tesco's were the golden child's of the back powder shooting fraternity, but not much else has changed.
