'safe to say' ... well, not really. The backup.mbz file is *copied* ... not 'moved' (there is a difference) ... so for a little while there the space occupied is about 2 times the space used by the course - 1 of those a dup in build area and the other (the .mbz file) compressed.
'remain stuck in ...' not a good sign ... go into one of those failed backup directories. Is there a moodle_backup.xml file and a 'backup.mbz' file? If not, no way of knowing where it failed ... and a course like that may never complete the building phase.
Depending upon version of Moodle, which you have not provided, there is an adhoc_task to cron (assume your cron job is running once per minute) that is supposed to try failed backups again as moodle DB hasn't been informed of successful backup for that course.
The build directories should not exist in moodledata/temp/backup/ ... there are clean up task which remove any failed backup directories ... or is supposed to be.
Did you check your servers error logs?
So ... rather than guess here ... share info about your system - Moodle version, DB flavor and version, where/how hosted, memory, free memory, PHP version, etc. etc.
'SoS', Ken