I havent actually installed KJ rears, but I've looked at KJ fronts, and they are tiny compared to a WJ.
I would think the problem lies somewhere else.
Somewhere else such as?
One note: If I bleed everything out real good, the pull a rear caliper, give the pedal a half a pump or so, then manually compress the rear piston back in and re-install, the rears work much better. Still crap, but they at least contribute to my braking somewhat. Still no where near being able to lock them up on even wet pavement.
I can't find any bulging lines (all steel and steel braided), and all 4 calipers are now brand new, as well as new pads/rotors. The other test I did was to have my buddy step on the brake as hard as he can. This locks the fronts, but the rears can still be turned by hand usually. While he's still on it, I cracked a bleeder on the front so the pedal would come down further, thus forcing more fluid to the rear, and sure enough, the rears locked up (at least to the point I coudln't turn them by hand, and they're capable of stopping my engine at idle.
The fronts work ridiculously well. On dry pavement in 2wd, they have no problem locking up my 35's. If I throw it in 4x4, the fronts will actually lock the rears through the driveline. This works fine on the trail, but on the street I can't be in 4x4, and ultimately on the street is where I really need the brakes to be working.
I've found fluid volumes, piston bores and strokes for every MC and caliper except the WJ and KJ calipers, and can't find anyone who's actually confirmed the bore/stroke of BOTH pistons on the XJ MC. Everyone says they're 50/50, but that I suspect is just a guess, as no one ever seems to post the actual bore/stroke.
I'll probably pull the MC tonight and tear it down to measure for myself, but even if I find that both MC pistons are equal stroke/bore, I still don't know the volumes for the calipers. For all I know, the KJ rear requires twice the volume of the WJ fronts. *shrugs*
