The JPC plate bag has a nylon front and a stretchy material on the back, with a velcro closure on the bottom. They are cut so that an ESAPI plate with kevlar backer is a very tight fit. A standalone SAPI cut plate would also be a very tight fit. A too-small plate would not sit still in the carrier as there is nothing that would hold it in place.
The way Crye sizes things is to determine the carrier size based on your body size, then they assume you are using the correct size SAPI plate. Based on your chest measurement you should be wearing a large JPC with large SAPI plates. If anything, what they are telling you is that your plate is too small. Buying a larger carrier won't fix that, it'll just create another problem in the form of a poor plate to carrier fit.
Don't worry too much about the plate size, 10x12 has been the standard outside of military armor for a while. Get a carrier that fits the plate and you'll be happier.