.blend files 'magically' shrinking? Still trying to understand some audio/video sync issues, I noticed the following weird behaviour (that seems to be related to said issues) of Blender.

Sometimes just opening a .blend file (not modifying anything, and I mean anything) and then exiting (without saving) will cause Blender to silently modify this file, unless it is marked as read-only. Moreover it seems the case that when this happens, the modified .blend file is slightly smaller than the original file (around 3 kB). Also it seems that this 'phenomenon' always occurs when I have these strange audio video sync issues.

Hence I would like to know what causes Blender to silently modify .blend files, and which data it is that is being modified.
Reopening your file removes all datablock that have been created and then not used, nor marked as "Fake user".

I.e. you create two materials for an object, then assign only one of them.
The unused object remains as datablock with no users.
When you close the file and then reopen it, the unused datablock (i.e. the unused material) disappears.

I don't know if this is your case!

See Blenders manual section on: Garbage Collection