"Optimize All Files Now" in OneNote not working

[Shan]

I'm using OneNote 2010. have a notebook of 33 MB with 7 sections. One
of the section has tens of pages with images. To reduce the file size,
I deleted the images, but the file size was not reduced. So I tried
the "Optimize All Files Now" a few times under file-options-save &
backup. The file size didn't change much either. I also deleted the
Cache files in case there were corrupted files, but it didn't help.
Does anyone know what's going on?

Thanks!