I don't exactly know what's causing your problem but there's a few things to consider:
a) the horizontal size pot is broken.
b) not sure if the horizontal width coil is broken, they're quite known to cease up and if you turn them, you can break them. on modern monitors with H-size pots you generally never have to mess with the width coil ever, unless of course someone else messed with it already. these are generally hard to find, cause there's no off the shelf replacement for them, you have to part them out of a dead chassis.
c) the L C and R pins are for adjusting the horizontal position. setting to L would bias its center point offset to the left, C dead in the center, and R biased to the right. under most circumstances you just leave this on C.
I've heard of mating 25" chassis to 19" tubes and it'll produce a huge blown up picture, or vice versa will produce a picture that doesn't fill the screen, but I don't think that applies between a 25" and a 27" situation.. most likely that's not the problem, and yours lies somewhere in the horizontal size circuitry. if you have an adjustment pot of the same resistance I would try subbing that in first.