In my experience, there's more difference between lots of the same maker's caps than in that maker's sizes. This holds when comparing differing manufacturers as well.
To put it succinctly: It's a crap shoot.
It only gets better when comparing nipple sizes and profiles.
When I first started in Muzzleloader shooting, there was a discount chain called "Jafco" based in Seattle. Just before they went belly-up in about 1980, they had a big sale to get rid of all their shooting stuff. I bought two bricks of percussion caps, one Remington and one CCI. Both #10's and both noticeably different in inside diameter and depth. Due to a pricing SNAFU I got them both for $1.29 for a brick of 1000. I called it to the manager's attention and he said it was too much hassle to straighten it out so, "Good Shooting". Both were darn good caps, too. I'm now working on another brick of CCI caps, of current production. They're good stuff, too.
Three Hawks