Good question. I never really thought about it before. ChatGPT gave a pretty good explanation:
Candy expands in a freeze dryer because of pressure and trapped gases:

• Air + moisture trapped inside: Candy often has microscopic air pockets and dissolved gases.
• Vacuum effect: When the freeze dryer pulls a deep vacuum, the external pressure drops dramatically. Any trapped gases expand (like a balloon in space).
• Moisture turning to vapor: Even small amounts of water or sugar syrups in the candy sublimate/flash into vapor under vacuum, pushing outward from inside.
• Structure softening: Sugar matrices (taffy, Skittles, marshmallows) soften slightly as ice or bound water leaves, so they puff up instead of collapsing.

That’s why candies like Skittles or marshmallows “puff” dramatically in a freeze dryer.