I could not find storage requirements for JSON type columns in MySQL documentation
How much storage space allocated for JSON type column in InnoDB? Are JSON columns the same as BLOB?
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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