Size of available free disk space after backup is finished is less than the amount of free disk space specified in Tools & Settings > Backup Manager > Settings > Start the backup only if your server has the specified amount of free disk space (in megabytes) option.
This is expected behavior. Because as stated in the option, free disk space checks before the start of backup.
Set a suitable value, estimating the among of space which would left after backup creation.
To check current disk space usage:
- Connect to the server using SSH.
# df -h