Since you can only assign user limits on a server-by-server basis and you cannot assign limits across all servers.
Also, you cannot assign limits in specific folders. It must be the entire server that your limit counts against. So . . . what happens if you update a spreadsheet last. Does that 20MB now count towards your audited limit? What happens if you create a small template SSheet for use. . . now you are the owner. Whose audited space does that count against?