What program are you using to do the backup?
You really need to find out what type of backup you are doing, a full back each time, or only backup changes.
You sound as though you are backing up everything every time you do a backup.
You dont really need to do this as most of the files dont change (the Windows files for example).
What you need to do is a full backup say every month or every two months.
Then backup JUST your personal files every so often (daily, weekly or whatever).
If you have been doing a full backups then you can delete some of the older backups.