SiteGround has set a limit for the maximum single file size to 8GB (other users reported lower values as 6GB and 4 GB). Creating a backup file on large sites might exceed that limit, and you will get an error message related to disk space issue like this:
The best thing to do in this case is to reach out to the SiteGround support team, and they should be able to raise this limit to 20GB or sometimes 60GB according to your site’s size.