This is my first post, so please be tolerantly and feel free to correct me :-)
Scenario describing:
I've got a SalesCollection('ID') with a DocumentCollection and I show the DocumentCollection in a table (sap.ui.table.Table). To bind rows i use the path "/SalesCollection('ID')/GetDocuments". And at least I'm binding the properties. 
That all works very well :-). Now i've got a new requisition. Every document of DocumentCollection has more details. I want to use one of this details in the DocumentCollection table. So I add expand to the table (see picture).
I looked at the browser debugger and the details are loading. Separately I checked the sendet URLs in my browser as XML => all details there.
My question is: How to bind the detail properties of documents. I tried: "GetDetail/DOC_SIZE" and "DOC_SIZE", but both don't work.
Has anyone an idea?