Understanding the x-ms-resource-usage in DocumentDB response header
I'm fairly sure the numbers are as follows, and all in KB:
documentSize: Size of the documentdocumentsSize: Combined size of all documents in collectioncollectionSize: Combined size of all documents in collection, along with overhead such as indexes
x-ms-resource-usage is about consumed resources within the collection, while x-ms-resource-quota is going to give you your limits. So with quota, you'll see documentsSize and collectionSize both set to something like 10485760, which is 10GB (10,485,760 MB).
documentSize and documentsSize are the same value - first one in MB and the second one in kB. Apparently, documentSize is being deprecated.
collectionSize = documentsSize+metadata (in kB)