Which backup method are you using? Is it based on archive bit or based on modified time? There must be some software who is modifying the files or making them appear modified to BE, because all we would do is refer the file as changed based on one of these properties. If any of this is found changed, a differential would back it up. Hence, please try and identify some application or some event happening on the server, which may cause the files to be changed and hence more data to be included in the backup.

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Thanks

Thanks for the reply.
  The full is set to backup files-Using Archive bit (reset archive bit.)
  The diff is set to back up changed files since last full-Using archive bit  (does not reset archive bit)

Would a wholesale permissions change reset the archive bit?

Yes, modifying NTFS permission does set the archive bit on. It is the operating system who decides about setting or unsetting of archive bit. So, if BE finds the archive bit set, it backs up the file. OS sets the archive bit whenever there is a change coming to the file system. You can try a couple of tests to understand the same. In order to identify the archive bit, you can go to properties of file and click on "advanced" button. If the option "file is ready for achiving" is set, you know archive bit is turned on else it is off. If you change the file, the check would be turned on. This check would be turned off by Backup Exec after full backup.

Thanks