There are two parameters in PHP related to uploading files. You have only one set.
The other is Maximum HTTP POST size.
In php.ini look for post_max_size = and make it same as the other parameter.
Comment: That's large!!! To be expected.
What you might consider doing is setting up file system repository in a single course accessible by system admins only. The file system repo is a folder in /moodledata/responsitory/[largecoursebackups]. You manually create the folder 'largecoursebackups'.
Then use scp or FTP or whatever to get those large course backups to that directory:
/moodledata/responsitory/largecoursebackups/
Then, in the course using that file system repo, one can restore those backups to other categories in Moodle.
Other things one might have to tweak ... max_allowed_packets in MySQL. Depending upon what's in those backups.
'spirit of sharing', Ken