From the docs:
Available Grunt commands
grunt dev— Compile LESS to CSS, concatenate and validate JS
grunt watch— Compile assets when file changes are made
grunt build— Create minified assets that are used on non-development environments
If you set define('WP_ENV', 'development'); in your wp-config.php then Roots will not use minified assets.
You can make grunt watch create minified assets if you like, see this post: Add x.min.css & x.min.script to watch - #4 by cfx