ms sql differential backup size jumps
Perhaps you've got a maintenance job - such as reindexing - that occurs once a week - that would cause a massive jump in diff backup file size. Or perhaps it's a once-a-week application job that imports or changes data?

If you're also taking regular transaction log backups (say once an hour), if you look at the file size of those backup files it may give you a clue as well as to when the change activity is.