If a dragon or similarily large creature swallows another creature large most D20 deviats have rules for being in the stomach of the creature and how you can try to get out. BUT there is one case I'm not sure if there are any rules for it, or how it could be handled:
What happens if the creature transforms into a smaller form while you are still inside? Do you still have the ability to get out and come out full form form the body, does the body explode, do you die,... ?
The details of size-changing magic (including enlarge/reduce, polymorph, and Wild Shape) are not often fully explored by the rules, leaving a lot of corner-cases where the DM will have to decide how it works.
When such an effect is mentioned within the rules, it generally either specifies that the creature's size changes as much as it can given the limits, or that the effect fails if there isn't room for it to complete. Some other spells that involve occupying a space you did not previously occupy (such as etherealness) instead shunt the creature to the nearest space that can hold it.
It seems to me that there's a tendency to treat size-changing as some sort of unstoppable force that can crush creatures to jelly or destroy even thick stone walls that get in the way. I think that's a mistake. There is no rule or metric for determining how strong a spell's size-changing power is as compared to material barriers, but it's a bit absurd to suggest that a low level spell like enlarge/reduce should be able to deal more damage through its size-changing than a purpose-made attack spell of the same level, and assumes the spell or effect has no built-in safety features that intentionally halt potentially dangerous shape-changes.
Personally, I would rule that size changing or form-changing magic fails if you try to use it while there's a Swallow-type ability still running, but forcing the creature to end its swallow effect is certainly also a valid choice. Deciding that this instantly kills either participant is probably not warranted.
Since swallowed is not a condition and in 5e only described in the action of the monster using it, there is room for interpretation, e.g. from Giant Toad:
If the toad dies, a swallowed creature is no longer restrained by it and can escape from the corpse[...]
There is no official rule/ruling as far as I know on polymorphing a creature that swallowed someone, so there are several options.
That when a creature polymorphs into something smaller than itself, e.g. large --> medium the swallowed creature could make a saving throw of some kind to be regurgitated with the DC dependend on the CR of the monster and the amount of change e.g. from large to tiny the DC gets easier.
Automatic regurgitation when the monster polymorphs, since it loses the swallow action and can therefore not keep another creature swallowed independent of size and other factors. (If the new creature also has the swallow mechanic it could stay swallowed).
Instant death of the swallowed creature, since it gets crushed in the stomach of the new monster.
Personally I think a combination of 1. and 2. is the most sensible option, since it is mostly easy to handle for the DM and gives a clear condition for the players.
