I have recently joined an organisation that likes to use Excel as a page
layout application (joy). They have been copy/pasting images into Excel -
hence the large file sizes - I have tried 'Compress Images' option (making
sure that I am applying to all images), and have also tried 'File', 'Save
As', 'Options' & 'Compress Images' and also turned off any version tracking
to try and reduce size.

This works in 50% of cases, but there is a whole bunch of them that refuse
to reduce in size and when re-opening the document, still report high
resolution images.

Why is this? Any ideas/Experiences?