ms sql differential backup size jumps

We are doing a full backup once a week and a daily diff between that. The total db is 80Gb, the diffs start out under 1Gb, then gradually grow to approximately 3-5Gb which is ok, but then all of a sudden jump to 40Gb almost every week. Why would this happen? How do I investigate? I'm reasonably sure that the actual db usage is uniform w/o spikes.

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.

StackExchange Database Administrators Q#1814, answer score: 7