SQL Server 2008 R2 differential backups much larger than expected
A small transaction log doesn't mean the diff won't be small though.  A diff backup looks at the physical page level since SQL Backups backup database pages.  When even 1 bit of data is changed in a page any point after a full backup, a flag is made active so the DB engine knows to back it up if a diff is taken.

Transaction logs largely record the minimal data needed to get the data within the pages back to how they were prior, thus it could be much smaller.  If you only changed 1 bit of data you are going to only reflect that change in your log.  When the diff backup runs, the entire 8k page is backed up.

Thus a Diff backup will ALWAYS backup the entire page if even 1 bit is changed on there.  A transaction log will record what's needed to roll that page back to the prior transactionally consistent state thus in this case it could be tiny, but the diff backup will still backup the entire page.

If you want more info check out the Backup Internals video on the Technet MCM site:
https://technet.microsoft.com/en-us/dn912438#mcm-readiness

Do you have any logging?  If not check out your default trace.  It'll at least tell you some minimal information.  If you don't have any logging, this would be a great time to start.