Is there a spell on the Witch's spell list which can shrink a treasure horde or store it in another small object?
If there isn't, I would suggest designing your own and asking the GM if you can use it. Research & Designing Spells
If it were me, I would suggestion something with duration (hours per level or ever 2 levels) not sure what the level would be or the cost, if any. It does seem like it would be useful, shrinking the size of the loot so you can carry more, but I'm not sure anything in core lets you shrink the loot.
Ant Haul is something to look at for if it's a weight issue, but it isn't on the Witch list.
It's not on a witch spell list, but Treasure Stitching is a good one if you can research it independently or find another way to get it on your spell list.
Is there a spell on the Witch's spell list which can shrink a treasure horde or store it in another small object?
You mean like secret chest (5th level Witch spell)?
You mean like secret chest (5th level Witch spell)?
So there's no shrink object spell or something like that? If not, ugh. Shelling out 5,000 gp on a material component. Good thing it's reusable.
load up a henchling or bystander with treasure then baleful polymorph. small turtle probably best choice. Then put in other container(cage/bowl/etc). Remove effect when you want your loot back.