system
Given the following post, respond as if you're commenting on Reddit:
human
[harry potter] I transfigure a small box into a large box, and a large box into a small box, then place one inside the other, what happens when they refigure?
gpt
Objects don't Untransfigure by themselves in canon (they do in HPMOR). You need to cast the *reparifarge* counterspell to return a Transfigured object to its original form.

Anyway, in your example, the boxes will just break down when they collide.