Just check what kind of app is consuming so much and optimize (e.g. for Preview App) or remove it. For me previews are most biggest folder, but it is already much smaller than before.
du -h -d 1 /var/nextcloud/data/appdata_xxxxxxxxxxxx/
64K /var/nextcloud/data/appdata_xxxxxxxxxxxx/external
177M /var/nextcloud/data/appdata_xxxxxxxxxxxx/ocr
31M /var/nextcloud/data/appdata_xxxxxxxxxxxx/facerecognition
24K /var/nextcloud/data/appdata_xxxxxxxxxxxx/cms_pico
204K /var/nextcloud/data/appdata_xxxxxxxxxxxx/theming
148K /var/nextcloud/data/appdata_xxxxxxxxxxxx/identityproof
15G /var/nextcloud/data/appdata_xxxxxxxxxxxx/preview
8.0K /var/nextcloud/data/appdata_xxxxxxxxxxxx/dashboard
1.3M /var/nextcloud/data/appdata_xxxxxxxxxxxx/dav-photocache
1004K /var/nextcloud/data/appdata_xxxxxxxxxxxx/avatar
620K /var/nextcloud/data/appdata_xxxxxxxxxxxx/js
3.5M /var/nextcloud/data/appdata_xxxxxxxxxxxx/appstore
8.0K /var/nextcloud/data/appdata_xxxxxxxxxxxx/richdocuments
12K /var/nextcloud/data/appdata_xxxxxxxxxxxx/text
1.2M /var/nextcloud/data/appdata_xxxxxxxxxxxx/css
15G /var/nextcloud/data/appdata_xxxxxxxxxxxx/