Our goblin druid today asked a question from which our DM blue screen was created:
"If I turned into a giant toad, swallowed a medium sized creature and then turned it around, what happens?"
The goblin druid is a small creature and the giant toad is a big creature. The creature that is swallowed is medium sized.
This is essentially the reverse of this question, even though it is not really a duplicate, since it is the eating creature that makes the shapeshift more than the eaten one. Closely related to this question, with the main difference wild shape from a willing druid vs polymorph an unwilling monster.