JSON storage requirements for MySQL
JSON have optimised storage format compare with old method (save as TEXT), but no any other information You will not find, 

because JSON size always depends from JSON data, some of them could be more optimised, some less, plus different length, plus ... 


  Optimized storage format. JSON documents stored in JSON columns are
  converted to an internal format that permits quick read access to
  document elements. When the server later must read a JSON value stored
  in this binary format, the value need not be parsed from a text
  representation. The binary format is structured to enable the server
  to look up subobjects or nested values directly by key or array index
  without reading all values before or after them in the document.


It is like - what zip archiver ratio? Is it same for txt and jpg?

some more information there - https://dev.mysql.com/doc/refman/5.7/en/json.html