Time Machine thinks the size of my backup is bigger than the source I have a new MacBookPro 16&quot; with a 500Gb SSD. Currently it has 332Gb of files on it.
I'm trying to run the first Time Machine backup, and it keeps failing because there's &quot;not enough space&quot;.
By default, it estimates a full backup to be 674Gb.

Even if I exclude the entire source drive, it reckons it will take 353Gb.

Both drives show no errors when analysed with Disk Utility. Anybody know what might be going on?
I eventually resolved this, though I'm not sure exactly what the issue was. Steps I took:

Wiped and reinstalled the OS using a SuperDuper backup.
Initially, Time Machine then showed the 'correct' size estimate, but then quickly changed to the incorrect version again. At this point, it was also showing me a phantom Time Machine backup drive that I couldn't remove.
Now suspecting some corrupted settings, I switched to my admin account. Having wiped my SuperDuper backup to use as a Time Machine drive again, I tried to set up Time Machine there instead. Here, I could remove the phantom drive, and the size estimates looked correct.
I switched back to my user account, and added back my TM exclusion folders. Everything still looked good.
First backup completed successfully \o/

It's possible trying to sort things out in the admin account may have worked in the first place, without the full reinstall. I guess we'll never know...