Automatic rezip to save disk space
There's a somewhat known trick to shrink down an XLSX, XLSM, or XLSB file: change the file extension to ZIP and unzip it to reveal its contents. Then select these contents and rezip them into a ZIP file. Change the extension of this new file back to the original extension of the original file. This new file will take up less space than the original file and still work the same way. However, once you edit the file, the space savings disappear.
My suggestion is for Excel to incorporate these space savings when saving a file without having to rezip the file manually.