So with SU2021 the file format was reworked.
I’m just starting to move to SU2021 now and compared the file size of a previously modeled project with the exact same contents but saved into the new file format. I need to do some changes to this project so decided to use it for comparison purposes.
I have to say you can colour me impressed, as the exact same model file comes in at just under 39% of the original file size.
Thank you for this bit of development SketchUp Team.