We had similar issue reported by the users. Agree with @EchoDu_MSFT, "% of Parent" in column storage metrics indicates the amount of storage utilized by a Component (Site/List/Library/Folder/File) with respect to total storage of parent(Site/Library/Folder).
In your case if you drill down to Documents, "% of Parent" column will
show the percentage of storage used by each file/folder relative to
Documents library. You can ignore warnings and continue to use library.
By default, you do not have any limits for storage at a site level and you have 25 TB storage quota allotted at site collection level.
However if you want you can set/manage site quota at site level as well, refer below article for more information.