Nextcloud iOS app "Documents & Data" size?


Having just upgraded my iPhone and trying to slim the old one down before taking a backup, I noticed the iOS app using a huge amount of space. Well, huge, compared to what I would expect it to use.
I went into the “Settings > Advanced” of the app and tried the “Clear cache” option a couple of times. And it sort of worked.
Going into the iPhone settings and choosing “General > iPhone Storage” and then choosing the Nextcloud app, the “Documents & Data” size went down from 30 GB to now 13.75 GB. But why is it using so much data even after clearing the cache?
Next step could of course be to delete the app and then reinstall.
App version is 2.23.3. Nextcloud server version is 15.05.