Huge snapshots with no clear reason

The 'lsof' command may let you know what files are being used while you are mid-workflow thus helping you locate the cache / working folder locations for those Apps. Activity Monitor can also show all open files for a particular process.

My first guess would be Adobe Creative Cloud caching and perhaps Avid does too. You could exclude those cache directories if you can find them. In the Time Machine Preferences -> Options you can exclude them from the backup but I guess the snapshots would still capture the data.

Another thought, perhaps you could move the caches to a scratch disk and that would get them off your primary internal disk. Or maybe even a partition on the internal disk.

Interesting.

In my case, I store backup disk images (up to 55GB) on the system disk which are normally excluded from Time Machine backups but (as I now understand) not from local snapshots. I wonder if this could explain certain Disk Utility errors (Error 60) which could result from automatic purging of local snapshots when the free space on the system volume is low.

The Terminal command lsof will list currently opened files which could be helpful in this case.

Good APFS explanation for the uninitiated:

The Electric Light Company (Blog) - Howard Oakley

https://eclecticlight.co/2021/11/13/understanding-snapshot-data-in-disk-utility/

A deep dive on everything APFS related:

The Apple Developer APFS Reference:

https://developer.apple.com/support/downloads/Apple-File-System-Reference.pdf

My thinking was that there could be large files (open and being modified) which could contribute to the large size of the snapshot. I had presumed that such files could be excluded but I see from this thread that local snapshots include all files so no impact on snapshot size.

I am a bit surprised about exclusions. For example, what if the Time Machine exclusions list lists multiple external volumes? Including them would certain balloon the size of the snapshot.

Reading more carefully, I understand from above that the local snapshots are taken of the full system volume without any exclusions but do not include external volumes regardless of Time Machine System Preference options.