Python string compression, why did the size change after concatenating with another string?

Question

Python

There is a dict structure, I distill it into a json string, I compress the json string, and then I collect a new string through string formatting. Why did the size of the final line increase by 3 times?

import json
import zlib

dict_data = {
  'command': "send_files",
  'files': ['images/elements/82.gif', 
        'images/elements/168.gif', 
        'images/elements/219.gif', 
        'images/elements/2yntZymm.png', 
        'images/elements/RGAlKgXS.png', 
        'images/elements/vedALO6A.png', 
        'images/elements/166.gif', 
        'images/elements/165.gif', 
        'images/elements/128.gif', 
        'images/elements/192.gif', 
        'images/elements/183.gif', 
        'images/elements/196.gif', 
        'images/elements/11.gif', 
        'images/elements/108.gif', 
        'images/elements/21.gif', 
        'images/elements/148.gif', 
        'images/elements/32.gif', 
        'images/elements/143.gif', 
        'images/elements/2vKwklHg.png', 
        'images/elements/106.gif'
                 ]
}

json_data = json.dumps(dict_data)
print("Len json_data: %s" % len(json_data))
# Len json_data: 592

compress_data = zlib.compress(json_data.encode('utf-8'))
print("Len compress_data: %s" % len(compress_data))
# Len compress_data: 165

result_string = "{'compress_data':%s}" % compress_data
print("Len result_string: %s" % len(result_string))
# Len result_string: 495

I was hoping the result would be (~ 18 + 165) bytes
Why this behavior?