I think that Christopher was referring to setting the location of the system zip binary in Site Administration : Server : System Paths. However, though this is something that was settable in 1.9 there is no way of setting the path in moodle v 2.4 at least.
I think that your original problem has to do with the gigantic size of the course backup. A course backup of 1.8Gb means that you either have a lot of media files or you've included something else by accident. I have found in the past that sometimes in a 1.9 installation previous course backup zip files may get moved outside the 'backupdata' folder and then they'll get included recursively as it were into the current backup. Personally, I'd delve into the course in question and either prune uneccessary files or zip up folders/files to move separately. In other words, you're going to have to handle course backups of 1.8Gb size as special cases!