Because everytime you backup, it writes a whole new set of files, not just updateing what has changed.
Go into Windows BackUp settings, and look at all the backup files you have, and the dates, etc. You can choose how much, how many, how often, etc.
It not only backups up Windows, but your Libraries as well.
There are other backup utilities out there, but I use Microsoft SyncToy 2.1 to do manual backups. I can choose exactly what to back up and where, and then "sync" the changes between drives when I run it.
Windows and Norton backups do very little, per my settings, as my Libraries are the most important things to back up. Other things I choose to back up are Contacts, Favorites, and my Outlook *.pst file(s). I don't need 3 copies of everything.