I bet Ben is correct.  It is more then likely the amount of foam and the teeth that are at the back of the mouth.  Not really anything you can do at this point unless you can detach the fabric at the cheek from the fur and get in under the fabric and trim away some foam.

Yeah, im talking about the cheek area, i have pulled apart puppets before just to alter bits and pieces, then put them back together and sold them to very happy customers! Unfortunately i dont really know if it IS the problem or not, the only way i could tell is if i actually had a play with the puppet and that isnt going to be possible, i really hope that my suggestion is the remedy to the problem, but unfortunately, like shawn said, the only way to figure it out, would be to detach the fabric from the cheek and get in there to see what trimming down you can do, other than that i cant think of another way, but all it takes is a little pulling apart, redefining and then gluing back together, there is nothing wrong with that and i bet you could still sell him, he is an awesome looking puppet after all!!
you will have to tell us what you decide to do,
Ben.