by Iñaki Arenaza.
Hi Visvanath,
Why there are so many users in the backup?
Because you told moodle to include the course users as part of the backup (see attached image).
Just select 'Keine' ('None') at they won't appear in the backup file at all. Beware this disables the backing up of Exercise and Workshop activities (in Moodle 1.9.x), as Moodle tells you in the next screen of the backup process.
Saludos. Iñaki.