hard drive – Big Sur: Time Machine couldn’t complete the backup

I keep getting the following error in Time Machine:


So I am backing up my OS drive (Macintosh – HD) to an external drive (EsEsDe), and I’m being told that the backup cannot be completed because my OS drive is nearly full – there’s plenty of space on the backup disk.

Is this expected behaviour? If there’s room on the backup disk, why should it matter that the disk I’m trying to backup is nearly full?

Is there a way around this other than the obvious (which isn’t really an option for me atm)?

Thank you!