Thanks Yoc.
I use GIMP (GNU Image Manipulation Program) to crop, rotate, and resize my scans before submittal, but even though Love Confessions No. 2 was only 1024 pixels in width, it still seemed a little on the large size compared to all the other books on the site.
I've just submitted Love Diary No. 36 (Orbit) for approval and changed the GIMP Quality setting from 92% (default) to 75. This brought the file size down to 28M which seems more in the normal range for a standard 36-page comic book. I could not tell the difference when I reviewed the pages so I might be able to go even lower.