Running Openhabian, just updated from 2.4 snapshots to release.
Before I did the upgrade. I did openhab-cli backup. I noticed it was really fast compared to previous backups and the resulting zip file was much smaller.
Noting has changed since the last backup, so, is that why it was fast and small?
It no longer includes cache, tmp and a couple of other directories which aren’t needed to be contained in the backup.
Ok, thanks. Good to know. 
I’ve noticed this as well. For a full backup you can use sudo openhab-cli backup --full command.
What’s openhab-cli backup currently backuping?
I’m not sure but as the OP mentioned it seems mighty small. I prefer to not take any chances and use the openhab-cli backup --full just to be safe.
The size dofference is Kb without --full and Mb when using --full.
The file it creates is just a zip file I believe. Open it up and see what’s inside and compare to what you have in /var/lib/openhab2.
For the most part, it only backs up config files which is why it’s so small. Binaries don’t get backed up unless you do a full. On a restore those binaries will get downloaded again.
And to put your mind at ease, when you clear the cache or upgrade using apt, all of those same files that are not included with the standard backup get deleted.
@h102 Thank you for the hint.
The regular backup on my Pi running 2.3 stable was around 109MB, I started the regular backup on 2.4 stable and got a file of 3.6MB 
- but with the “–full” option on 2.4 the backup file size is in the old range (now around 115MB)…
However as @mstormi pointed out there might be a couple of directories that are not needed. But I wonder why they have been included as standard backup in 2.3…
Thanks for clarifying!