I’m using WD Backup under Windows 10 with a 4TB My Passport. For files where the name includes a ‘.’ it keeps the last 5 versions in the History folder. But for files with no ‘.’ in the name it keeps all versions, i.e. never deletes the oldest.
I’m using Thunderbird and its mailboxes have that kind of name; the inbox in particular is 4GB and I’ve got the backup set for daily, so it uses another 4GB or more every day.
This must be a bug in WD Backup. I had the same problem with the older version (that didn’t have the separate history folder) and a 1 TB drive under Windows 8.