I know this is an old thread but I've just encountered the same issue.
Someone had used a Maintenance Plan to backup the databases, Full at midnight and Differential every 4 hours during the day.
Both backup files were the same size, actually the Diff was a little bigger and growing.
Running an independent backup of each type showed the Full was big and the Diff was tiny, as I had expected.
Whilst this might not be the problem you are encountering, it is worth checking this....
When I looked more closely at how the Maintenance Plan had been set up I could see the Diff Backup was only that in Name and it was actually defined as a Full backup.