For your PDF files, what does gs do or not do which reduce the PDF file size? Is it mainly ebook option?

Yes, I've found the ebook option a convenient way to downsample the resolution of the images embedded in a PDF to 150 DPI. So this script will have a significant effect on PDF files that are large due to them containing high-resolution images (like scans), and less of an effect on PDFs that are already smaller than ~1MB.