No hints. Only questions and suggestions. Cause it's strange.
Operating system is? Remotely hosted? Shared system or dedicated/VPS's?
Are the backups going to 'File Area' or to a designated directory? 'File Area' is the sea of files in moodledata/filedir and thus makes deeper investigation a little more complicated as finding the backups require an SQL query to find the contenthash - thus location of the backup file.
That report of automated backups is generated from tables in the DB so this could be related to some meta data mis-calculation ... thus the following ...
Suggestion: go to one of those courses and backup via Moodle Course menu. Step through the screens during the backup process to see what's automatically checked for backup. Is the backup file the same size as what you see in the autobackup report?
Further investigation requires some command line foo but think I'd compare the questions.xml file size from two backups (this is why easier if backups were sent to designated directory).
So let's say there is a backup file in a designated/test directory by this name:
You can extract from the mbz the questions.xml file and the moodle_backup.xml file from the backup without extracting all the contents with this command:
tar zxvf backup-moodle2-course-104-dso_intro-20181205-1612.mbz questions.xml moodle_backup.xml
Issuing the above command, will display:
ls -l *.xml
-rw-r--r-- 1 root root 25589 Dec 5 11:14 moodle_backup.xml
-rw-r--r-- 1 root root 5760253 Dec 5 11:14 questions.xml
xml files are ascii so you could open with a text editor to view ... moodle_backup.xml good candidate for that ... or search it for quiz hints:
fgrep 'quiz' moodle_backup.xml
suggest renaming questions.xml to match the course name ... dso_intro_questions.xml following this example).
Then do the same thing for another backup of the same course that's larger.
Are the questions.xml files increasing in size?
'spirit of sharing', Ken