why the local .js file size is not the same as browser request back from server?

I used react and webpack to build up a .js file like below.

5.9M ./app.8d30cc2e71548cd03a7b.js

but when it is uploaded to the server and then visit from browser. The network display like this.

the same file, why the size change a lot, amazing.

what happened in this process?