Which one is modern light weight message exchange format?

The data model is different. Apart from the lack of markup, which you noticed yourself, JSON also doesn't have all the other XML features, such as:

Processing instructions

Comments

Attributes

Hence, not only the data itself is more lightweight, but also any library implementing the data model.

This makes rendering, parsing, processing JSON with non - JavaScript languages a lot faster. For JavaScript, this is a no - brainer, as JSON is actual JavaScript code, and thus much more "lightweight" for a JavaScript parser to process