Recently we started noticing that out TFS database started growing rapidly (nearly 100 GB per month). On further investigation we found that the tests that were executed as part of a CI build had some big files that were being uploaded to TFS.
Grant Holliday has a good blog on using the test attachment cleaner to remove these big attachments – http://blogs.msdn.com/b/granth/archive/2011/02/12/tfs2010-test-attachment-cleaner-and-why-you-should-be-using-it.aspx
We used the tool to clean up a lot of database space. For preventing further growth of the database we also installed the hotfix (http://support.microsoft.com/kb/2608743) on all our build boxes.