oC needs 3 times as much space on the server?

I run Owncloud on my laptop (which is currently my only computer) to sync newly downloaded music files to my NAS. Firstly to back them up, but also because I use the Owncloud directory on the NAS as a base to sync them into my smartphone (via Foldersync).

Now that I try to make a backup from the data on the NAS I noticed that the directory oC "resides" in on the NAS is more than 3 times as big as the data on the laptop. It's 9.65 GB there, but the File Station on the NAS shows a size of 29.21 GB.

Is this normal? If not, how can I find and resolve the reason for this gross "blow-up"?

Obviously, I don't want to waste space on my NAS (and neither on the backup-NAS)...

In case it plays any role:
The Owncloud version on the NAS - a QNAP TS-459 Pro II running QTS 4.2.2 - is ownCloud 8.0.4 (stable). I downloaded it from QNAP's appcenter (as I don't know enough Linux to do an installation from the command line). There's no newer version available in the appcenter.
Owncloud version on the laptop (which runs Win7) is Version 2.2.2 (build 6192).

The trash-bin and files versioning can take some space.

Version 8.0 is not supported any more. You should at least use 8.1.11 (https://owncloud.org/changelog/#latest8.1) but also plan to further upgrade since the support of this version is running out next. Or ask the person who supplied the owncloud app on qnap to provide an upgrade.

Don't update to version 9.0 or newer, there are some software version needed that are not provided by the QNAP OS: https://central.owncloud.org/t/upgrade-to-9-1-on-qnap/3803

Could also need an update.