Source files after building the react app are not minified. both js and css files

I have run “npm run build” within my react app. after that i was serving the files using “serve -s build”. And in my environment files i had disabled source-maps by adding “GENERATE_SOURCEMAP=false” to my .env file. but when i open the source tab in my chrome dev tools, both the js and css files are not minified. they were shown with indentation as below:

i wanted to show minified files only. Any help would be appreciated.