Okay, I've tested saving the same .c4d scene file (7.88 GB) in R21 and in R27 to the shared network folder.
When loading the scene file in R21 it took around 11 minutes to load.
Saving this scene in R21.207 also took around 11 minutes (7.88 GB).
I then opened the scene file in 2023.1.0. (R27).
Saving the file was instant (less than 1 second!), but the file size ended up just being 12.6 MB.
So, I opened this scene file saved created by R27, in R21 and everything seemed to be present, but when saving again, the file size ended up being 12.7 MB.
Now, I tried the old trick, to copy all objects into a new scene in R21. I did that with the 7.88 GB file and saved. Now, I get 12.3 MB.
Well, I can't draw a conclusion from my findings.