The size of the course not effected here, because we already got backup of one course it has a higher size than this one.
Also, I have tried to create backups by unchecking all options from the backup page (Like userdata, questions, other data) but again repeat the same.
I have a doubt that Moodle course needs any capability or any other extra settings for create backup??
I compared the course settings with other courses. But can't find any changes.
What do you think ?? Thank you very much for your valuable reply