There is a comparison table at the end of the project's README (https://github.com/afiskon/zson). However, the table columns are not explained very well. It is not 100% clear to me whether "before" means "uncompressed" or "PGLZ compressed".

  Compression ratio could be different depending on documents,
  database schema, number of rows, etc. But in general ZSON
  compression is much better than build-in PostgreSQL
  compression (PGLZ):

     before   |   after    |      ratio       
  ------------+------------+------------------
   3961880576 | 1638834176 | 0.41365057440843
  (1 row)
  
     before   |   after    |       ratio       
  ------------+------------+-------------------
   8058904576 | 4916436992 | 0.610062688500061
  (1 row)
  
     before    |   after    |       ratio       
  -------------+------------+-------------------
   14204420096 | 9832841216 | 0.692238130775149