Identify Causes of Large Time Machine Backups
Time Machine, OS X 10.5's built-in backup solution, normally runs reliably and trouble-free for me. So much so that it has become my always-there backup solution, and I rarely think about it unless I need to retrieve a recently-deleted file from the archive. Sometimes, though, Time Machine seems to lose its mind, and instead of small, fast hourly updates, I'll see hourly runs that take many many minutes to complete. Beyond the time required for each hourly run, this issue meant my Time Machine drive was rapidly filling with data, leading to a shorter history for keeping multiple copies of files.
One possible cause of repeated large hourly backups is TechTool Pro 5's Directory Backup feature, which creates a copy of each hard drive's catalog (a file that tracks the files on the drive). I found out the hard way that if you have this feature enabled (it's found on the Directory Backup tab of the TechTool Protection System Preferences panel), you will get continual huge hourly Time Machine backups. In my case, I was seeing in excess of 2GB on each and every backup.
You can either disable this feature (on the previously-mentioned Directory Backup tab), or alternatively, exclude the folder that holds the backups from Time Machine. (If you choose this solution, you'll want to regularly back those files up to another device; backed up catalog files aren't much good if the drive they're on dies.)
To do this, open the Time Machine System Preferences panel and click on the Options button. In the window that appears, click the plus sign, then select your user's Library -> Application Support -> TechTool Protection folder in the file selection dialog. Click Exclude, then click Done to make Time Machine leave this folder out of its backups.
What if you're not sure about the cause of your large backups? You can keep an eye on the backups yourself, or use any of a number of third party tools, including Time Tracker, timedog, or Macworld Gems winner Back-in-Time.
While it'd be nice to get more information directly from Time Machine about what's happening with backups, the above tools can help you figure out just what went into any given backup. With that information, you can then decide whether or not to exclude the offending files and folders from future Time Machine backups.