[Terri Bruns]

This fix worked perfectly for me:

When you Save a PDF with Illustrator, that is essentially an Illustrator document. It’s meant to be used as part of a PDF workflow, so it is saving as much information as it can.

If you want to make a shareable PDF with the smallest file size, choose File ? Print and print to PDF. That is a final PDF that is meant for distribution.

As found in
https://graphicdesign.stackexchange.com/questions/67497/adobe-illustrator-is-making-my-pdfs-huge-why