Cannot find reason why differential backups size reduced
The most likely cause by far is that a full backup was taken out of sequence (between that differential and the previous full backup) - that makes the differential smaller as there are less differences to include between it and this extra backup than between it and the expected previous backup.

If this is the case then to be able to restore the unexpectedly small differential you need to find a copy of the extra backup.

This is usually because someone has taken a copy for use elsewhere, perhaps for testing a support issue or new release. IT is important that everyone who has access to be able to take backups knows that they need to be careful of this because it breaks your DR plans and so forth.

There is a safe way to take an extra backup: use the COPY_ONLY option. This takes the backup as normal but does not interfere with your main full/diff/log backup sequence.