As mentioned by Richard Smith in the comment, 559 in the nginx log stands for:

the number of bytes in the HTML response that Nginx sent to the browser

Source: http://nginx.org/en/docs/http/ngx_http_log_module.html#log_format

or, as specified in the docs:

$body_bytes_sent
number of bytes sent to a client, not counting the response header; this variable is compatible with the “%B” parameter of the mod_log_config Apache module