Differential backups in SQL Server 2008 R2
Differential backups include data that has changed since the last full backup (ignoring any full backups taken with the COPY_ONLY option).

If you take a differential backup immediately after a full backup, it will be very small, as little (or no) data will have changed. As time goes on, the differential backups will become larger and larger, until you do another full backup, which resets the differential change map.

Note that you must still have the last full backup taken prior to the differential backup, or the differential backup will be useless.