[Closed] Windows 7 Backup Utility - question

Just been playing around with the backup utility that comes with Windows 7. Seems ok, if a little slow to do the check for changed files. Anyway, my question is, when does it decide that it can delete files from the backup that have been deleted from the computer?

I have set it up to backup certain files and folders (i.e. not the system image) so that I can access and restore individual files if needed. I deleted some from my hard drive and then ran the backup but it kept the deleted files. Great for if/when I delete something by accident and want to restore it but how do I get the backup utility to mirror the files and folders rather than just add any new ones? My backup will rapidly get too large if it keeps everything. Perhaps it starts deleting files after a set time?

Any ideas?