Checking the gui of nginx you will find that is NOT compressed. I dont know where did you see this but the standard is compressing only css (period).
I was comparing this part of the nginx setup with your finding :
... gzip on; gzip_types text/plain text/css text/javascript application/x-javascript text/xml application/xml application/xml+rss application/json; ....Then I found https://stackoverflow.com/questions/23939722/nginx-gzip-not-compressing-javascript-files : this explains what you are seeing - and what needs to be done ?