by Ken Task.
Don't think the error message has anything to do with version of Moodle (although newer version backups will probably not restore to lower version moodle site), but rather, just as it says ... 'uploaded file may exceed post max size in php.ini'.
If you are not the server administrator ... the operating system server administrator, not the admin of the Moodle, you might not have access to php.ini in order to set the size limits higher (check the size of the .mbz backup).
Please see: https://docs.moodle.org/27/en/File_upload_size
'spirit of sharing', Ken