Large incrimenal backups of PST files

Hi,

I need to backup PST files on an file server and the incrimental backups are looking quite large. I am being told that the changes in the PST files day to day are quite small (only a few emails).

Does anyone know how netbackup incrimetals go about getting the changes from pst files?

Will the smallest change trigger a lot of the file to be backed up or is netbackup quite intelligent about what it backs up.