Re: Seems obvious.
The question is whether the entire XML/JSON chunk gets passed over the network to the client. In some NOSQL type databases, the entire chunk of XML/JSON is always passed over, leaving the client software to extract the atrribute(s) required. Thus if you have a 4k chunk of customer XML, but you only require the customer name (say 50 bytes) then there is a huge difference in network traffic - 80x in this example.
SQL enabled databases will most likely have the 'projection' operation where only the required columns (attributes) are returned. Some of the more advanced databases will also allow you to apply a projection inside a JSON/XML structure.
Biting the hand that feeds IT
