We have several SQL Server 2008 R2 databases for which we perform a full backup every Sunday then differential backups Monday to Saturday. We also do transaction log backups every 10 minutes.
The first differential backup on Monday is usually quite small, but Tuesday to Saturday are much larger but similar in size to each other.
I used some scripts I found which predict the differential backup size, e.g. https://dougzuck.com/sql-differential-backup-size-prediction and http://www.sqlskills.com/blogs/paul/new-script-how-much-of-the-database-has-changed-since-the-last-full-backup/, and they predict a very much smaller backup size.
database1, full backup size 5Gb, diff size 3.5Gb, predicted diff size 84Mb database2, full backup size 40Gb, diff size 1Gb, predicted diff size 17Mb
As you can see, the actual sizes are hugely different and can be half the size of the full backup after only a couple of days.
I know users aren't creating or modifying the actual data to any great extent. As far as I can tell, there are no index rebuilds or other management tasks happening between the full and differential backups.
It's like something is happening on Monday which causes the Tuesday onward differentials to be huge. Backup compression is not used.
I looked at the transaction log backups and the average of the total daily size is around 40Mb. I know it's not as simple as this, but 7 days at 40Mb = 280Mb. Doesn't explain the 3.5Gb differential backup! There are no huge transaction log files. There are no bulk load operations taking place.