It seems that the app creates snapshots, which means that the size might grow until the server is full (not sure if the app does incremental backups or starts from zero for each snapshot).
- it’s incremental, similar to borgbackup / restic, the storage design.md has the explanation.
- snapshots are limited to 3 daily, 2 weekly. As you said, this is fixed in code and not a user config
- beware with nextcloud nginx+php-fpm setups (vs apache), see 0 byte files and chunked transfers, a long workaround is imo scheduling a sync of the Seedvault backupdir with RoundSync instead