Note: I just "flattened" the file, then saved at High Quality Print with a compression quality setting of Maximum. The file is now 9.5 MB. It looks the same to my eyes as the 113 MB PS file image.
Question: When I uploaded my cover a few days ago (an earlier version of the cover), the PDF saved at 39 MB, so I wonder if a 9.5 MB PDF suggests a quality that is too low.