I'd guess seal, but see if somebody can get you a
I've seen rubber seals expand. Funny thing is, brake fluid on the wrong type of rubber would do that, but obviously shouldn't to a brake part o-ring, but heat, or somebody stuffing the wrong o-ring in the bag with the right part number would be my number one guess.
Get somebody to measure a piston for you. Try a racing board (where someone may have their stock calipers off from a Brembo/stoptech upgrade) or maybe visit Pelican parts site and see if somebody there can give you the diameter spec.