Now I'll confess am NOT a certified MySQL DB Administrator ... am 'certifiable' but not 'shingled' to the hilt and don't care to be!!!! LIke you, in a former life, a classroom teacher interested in providing an online experience for students.
While one will eventually really need that knowledge, at present let's see if something else less 'heady' that will help without much effort.
There is a plugin called 'course sizes' which takes a long time to run but will list the 'largest' courses in your Moodle AND show an 'approximate' backup file size. It will provide a link to the course so you could go inspect that course. Large files could be tied to many things ... attachments to forum postings, for example. Another reason for inspecting courses ... you know who to contact should you have questions about how the course is setup/reason for X and could suggest alternatives to teacher.
https://moodle.org/plugins/report_coursesize
Other plugins:
For when you desire to 'test' your MySQL query knowledge/skills and gen your own reports:
https://moodle.org/plugins/report_customsql
Other related plugins:
https://moodle.org/plugins/browse.php?list=category&id=4
Caution about adding plugins ... one can overdo that ability. Too many plugins gives rise to potential for issues. Would strongly suggest setting up a sandbox Moodle implementation that's the same version as your production server, and use it to 'test' plugins. You could restore (or attempt to) those larger courses in the sandbox and work on alternative suggestions given to teacher ... even sign the teacher up as teacher in that sandbox course of theirs for them to 'kick the tires' there before making massive changes to their production course only to find out, behavior is not as expected.
Well, we've not attracted Usman's attention here ... he's a resident Windows 'promoter' and has provided many solutions to Windows folks.
One cannot assume the size of the course is due to files although that is one factor. If a course has tons of quizzes/test that too could be one of the reasons a course is difficult to backup. This has already been mentioned - a bug supposedly still exist.
As far as finding backups in mdl_files ...
select filename,filesize from `mdl_files` where (`filearea` like "%backup%");
Filename of a backup should give the course ID and it's shortname so you can locate courses that way.
'spirit of sharing', Ken