I'm getting errors trying to move some files into a SharePoint site of ours, and when I check how it's set up, I'm seeing the below, 
We've a total of 25TB space available, of which just over 2 has been used, but the Documents folder shows up as 100% for % of Parent. I don't really understand what the "Parent" is here.
I hope there's an easy fix, but if not I'll contact MS' support tomorrow morning.
The "% of Parent" column indicates the amount of storage that an item takes up relative to the rest of the items in that scope (i.e. that item's parent). In terms of the "% of Parent", it's the % of the parent's directory used. Drilling into the document folder will show the breakdown of usage scoped to that folder. The "% of Parent" column will show which files or folders are using the most space proportionally to each other only relative to that folder.
You can continue to save documents until the total usage of the site reaches the site collection's quota. Looking at which folders and items are using the most space (via the "% of Site Quota" column) can help you find and remove large documents and folders if you ever hit the site storage quota. The "% of parent" column can then help you choose which specific files to remove.
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.
Click here!
