It won't meld into the form because that only happens when Polymorph is cast, not when it expires. As you said, it is not covered so it is up to the GM!
The easiest solution is that the items fall to the ground around the creature.
It's not covered because it doesn't apply. The melding rule only applies when a caster takes on the form of an animal, dragon, elemental, magical beast, plant, or vermin type. If you're going from dragon to humanoid, there is no melding.
The best bet in this case is:
Size and Magic Items
When an article of magic clothing or jewelry is discovered, most of the time size shouldn't be an issue. Many magic garments are made to be easily adjustable, or they adjust themselves magically to the wearer. Size should not keep characters of various kinds from using magic items.
There may be rare exceptions, especially with race-specific items.
Armor and Weapon Sizes: Armor and weapons that are found at random have a 30% chance of being Small (01–30), a 60% chance of being Medium (31–90), and a 10% chance of being any other size (91–100).
In that case, I would resize most items except things where size is explicitly stated (weapons/wands/staffs/etc). For armor specifically, I'd have it fall off harmlessly.