by Ken Task.
Not sure anyone could respond to the question ... no two courses, unless one is an exact clone of the other, would be alike, thus the process for backing them up is also different. Size is but one factor.So suggest turning on debugging and attempt backup that 3.8 gig course (which isn't really all that large) again to see if Moodle will report something useful. In the temp directory used for the course backup, BTW, there is a .log file and if a backup fails that log file might give you a clue ... if one could figure out the numbers associated with the stages/plan - ie, backing files, or activities, etc.
Things to check:
PHP setting for memory a script can consume. Think default is 128M .... and the backup process will attempt to use even more. Server apache error logs, might show a line about 'couldn't allocate X bytes' is an indication the memory for a script to consume needs to increase.
PHP setting for time for a script to run ... fact that it gets close to the end of the process could indicate it's just timing out.
As a test, one could take apache out of the loop and run the backup.php script found in moodlecode/admin/cli/ ... just have to give the course ID as a parameter.
That actually uses php cli not the php apache uses.
Without debug or clips from server logs, anyone's 'guess'! ;)
'spirit of sharing', Ken