When backups are set to save in the course folder and course backup files are included in new course backups. If a large enough number of backup files are kept (for example 20), then course sizes quickly grow to several gigabytes in size.

In our particular case the client was then locked out of their account because they overloaded their quota.

This was on a Moodle 1.6 install, but I believe impacts many versions of moodle.