What this tool does
Keep text rewrites the PDF and drops unused objects. Selectable text stays. Many files will not shrink much that way.
Smaller file and Smallest file redraw each page as a picture. That often reduces size for scanned or image-heavy PDFs. The new file is no longer a text document.
When nothing is saved
If the new file is not smaller, the page says so. It does not pretend a reduction happened. Try the smaller-file setting, or leave the original as it is.