Thumbnails are generated to be the width as specified in the Index page (or the default of 1024), unless the image in the PDF/DjVu is less than that in which case it uses the max.

However, to calculate the max it is only looking at the first page rather than the specific page that is currently being edited in PRP.

For example,
https://commons.wikimedia.org/wiki/File:Hikayat_Pelanduk_Jenaka_Or._1932.pdf
is showing this as the thumb for page 9:
https://upload.wikimedia.org/wikipedia/commons/thumb/7/73/Hikayat_Pelanduk_Jenaka_Or._1932.pdf/page9-212px-Hikayat_Pelanduk_Jenaka_Or._1932.pdf.jpg

This is only an issue for files with narrower first pages, so it's reasonably rare.