Setup a custom disk-size for Nextcloud

I have a web-space with PHP where I have a Quota of 50GB for my data. Nextcloud shows me a total quota of several TB. Could I somehow correct the Quota shown in Nextcloud?
I do not wish to setup Quota per user because I wish to use the Diskspace as efficient as possible.

Set quota to e.g. 20 GB. Delete every Nextcloud upgrade backup because it uses much data. If system full it can destroy your Nextcloud.

I know How I could save space that’s not the Problem. The Problem is that currently I have fix Quotas per user on a low level and wish to deactivate the per user Quota.
And how could I set up such a global Quota for the whole Nextcloud?

Any Ideas?