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.