Even though it seems like in Finder views that a file appears in many different time stamped backup folders, there is actually only one copy of each unique file in the entire backup set. IOW, if you created 'myGiantFile' on February 1st & never changed its contents after that date, even though you see it in every time stamped backup folder created afterwards, there is really only one copy of the file on the Time Machine backup volume.
This is done using hard links, as described in http://pondini.org/TM/Works.html.
If you find this hard to believe, compare what Finder's "Get Info" says for the total space used on the Time Machine volume vs. the sum of the sizes of the time stamped backup folders. You should see that the sum of the folder sizes is much larger than the total space used -- sometimes it is even larger than the capacity of the drive.
Thanks for the reply! I'll describe the exact situation. The file is the Entourage database which is about 4gb. During the day, many changes happen to this file, and, if I have the Time Machine disk connected, I believe the entire file is backed up repeatedly. I understand what you're saying about how Time Machine displays the entire contents of the disk in every backup, but I think in this case it's actually making multiple slightly-changed copies of this file. (It'd be great if you could tell in backups which files were hard links and which were "real".)
What I'd like to do is keep one or a handful of backups of this file for safety, but I don't need a backup of every little change that happened. In other words, I'd like to thin out the number of backups of this file without deleting entire backups of the whole disk. Is this possible?
As I understand it, the Entourage database is monolithic so Time Machine must backup the whole thing to retain its integrity on even tiny changes to it.
I found http://hints.macworld.com/article.php?story=20091014152142736, which offers a workaround for this, but I can't say how well it works because I don't use Entourage.
The only other suggestion I have is to exclude it from Time Machine backups (in the TM privacy pane) & back up the file manually occasionally.
Thanks! Too bad Time Machine doesn't give you that control. I'll try the solution in that link going forward. And maybe what I'll do for previous versions is make a copy of the file, make sure Time Machine makes a good backup of that, then delete all backups of the original file (although I guess that's not too different from just doing what it says in the link and deleting the previous backups).