Backups of that small size is really not that unusual IF there were 'test courses' ... no students assigned and no activities ... just a shell course.
In 1.9.x on CentOS (other Linuxes as well) there was an issue of creating a large backup (over 4.0Gig) and when that operation was completed 'copying' the backup (.zip) to course ID/backupdata/. Because that was the last operation of the entire process, Moodle would record the backup failed. But it didn't really. When inspecting /moodledata/temp/backup/ one could find a completed/intact backup. Operating system just couldn't copy. Linux was capable, but one had to compile supports for such large files into the kernel. Uhhh, something I'd care not to do. One could, however, manually move the backup to it's proper location.
Check your moodledata/temp/backup/ folder for anything remaining.If the course backups reached that threshold there might be some .mbz files there.
Also, there might be .log files larger than 0 bytes. 0 byte files indicated successful backups. Any .log file larger than 0 bytes might also give a clue concerning backups.
'spirit of sharing', Ken