by Ken Task.
Actual backup file size can be mis-leading. So one cannot really look at that and say that this one or that course should backup faster.
Two big processing parts ... quizzes and assignments.
Here's 2 real scenario ...
Digital Photography class of 20 students. Teacher's assignment says to check out a digital camera and take 6 photos each of 6 different lenses/settings. That's right off a digital camera which means files will be somewhere between 1.3Meg to 2Meg - X 36 X 20.
Have you run a health check on your site? http://site/admin/tool/health/ If quizzes have issues that could affect backups.
Working with a sys admin right now who runs a moodle for home schoolers and they organize their courses full academic year. Some of those courses have over 100 quizzes in them. Server is powerful but some of those courses never finish backing up via automated backups ... must use single backup script from command line cause that takes apache out of the loop AND due to processing have even had to start the process ... kill it with parameter that will allow restart of the backup in a 'nicer process' and run until completed. That's something that can't be scripted and must be done manually.
On some sites have had to turn off automated backups and then create 2 bash shell scripts that uses /admin/cli/backup.php for a list of courses that are large and that take up processing (sometimes pegging the top output to 100% of CPU). The other bash shell script covers all the other ('smaller' / less intensive in processing) backups and run them via a cron job outside of Moodle.
'spirit of sharing', Ken