JSON is effectively serialized JavaScript in that you can eval(aJsonString) directly into a JavaScript object. Inside of a browser it's a no-brainer JSON is perfectly suited for JavaScript. At the same time JavaScript is a very loosely-typed dynamic language and cannot natively take advantage of all the specific type information available contained within an Xml/Xsd document. This extra metadata (which is great for interoperability) is a hinderance in JavaScript making it more tedious and cubersome to work with.
Size vs Performance
If you're in a browser JSON is faster to serialize/deserialize as it's simpler, more compact and more importantly natively supported. I have some northwind database benchmarks available comparing the size and speed between the different serializers available. In the Base Class Library Microsoft's XML DataContract serializer is over 30% faster than their JSON one. Although this has more to do with the effort Microsoft put into their XML serializer as I was able to develop a JsonSerializer that is more than 2.6x faster than their XML one. As for payloads based on the benchmarks it looks as though XML is roughly more than 2x the size of JSON. However this can quickly blow out if your XML payload uses many different namespaces within the same document.
