Cutting the media box of a file

Hello,

I am trying to submit a PDF to Kindle Direct Publishing (KDP) for a book. I scanned illustrations into a PDF and cropped the excess white space because the illustrations were smaller than any standard page size for scanning.

When I submit KDP is telling me that while my page dimensions are correct, there is another attribute they refer to as the "media box of the file" and have told me that is sized larger and causes my upload to be rendered at that larger size.

My question:

How do I shrink the "media box of the file" to match my cropped page dimensions? As I don't see anything other than the cropping I have already done.