Twice this week, I have faced a situation where *Polymorph* was going to be cast on a creature that had swallowed a PC. As a DM, I ruled that if the polymorphed form was smaller, the swallowed PC would burst out, causing damage to the creature. As a player, my DM ruled that the spell would simply fail. 

Are there any rules on what should happen in this scenario? The texts of *Polymorph* and the swallow ability of various monsters don't seem to address this situation. 

([Related][1], but for 4e)