Mini-humbuckers are pretty low-output (or to be more relevant, low-volume) pickups. I've never tried it, but I suspect just about any humbucker in the neck position would overpower a mini-humbucker in the bridge.
A humbucker in the bridge and a mini in the neck is fine, of course, but no good to you.
I'm a bit confused, though, what were the original pickups, a pair of full-size humbuckers? Did you use some kind of adaptor ring to fit the new bridge pickup?