I have a scanned PDF of about 16 MB that I want to removed over 75% of it. I've tried image/clip, image/crop, and edit/crop and the appropriate area is left but the overall new filesize is still 16 MB. Is it really removing the areas or just turning them to the background color?
Since you can revert the crop, I assume it will not remove the data from the PDF but just resize the "crop box".
You could try the options available in FILE -> Compress to reduce the file size or choose another document size in EDIT -> Size.