Time Machine automatically excludes hot folders like /var/tmp and /var/vm and Library/caches from its backups. But it does not exclude, by default, the Downloads folder.
If you are like me, things in your Downloads folder are mainly transient items that have not yet earned the status of permanent guests on your hard drive. So add this to your list of excluded items by using the Options button in the Time Machine System Preferences panel.
I invite readers to add comments suggesting other overlooked folders with lots of churn that should not be backed up by default.
Mac OS X Hints
http://hints.macworld.com/article.php?story=2008073014165984