When CSS is heading for production, it takes an extra step through cssnano (http://cssnano.co/). It's a fantastic and modular CSS minifier by the extraordinarily talented Ben Briggs. Highly recommended.

Besides the more obvious minification step that cssnano provides, there are a number of micro-optimisations you can perform on your CSS just by incorporating plugins from the PostCSS eco-system. For example, by consistently ordering your CSS declarations, Gzip can compress the style sheet more effectively. That's not a job I want to do manually but the postcss-sorting (https://github.com/hudochenkov/postcss-sorting) plugin can do it for free. Here's comparison of Gzip file sizes using the various declaration sorting configurations.

To exemplify, I took a large test CSS file, and unsorted once Gzipped it was 37.59 kB. Here are the file sizes of that same file when Gzipped after using the other declaration sorting configurations:

So at best we gain a saving of just under 1% of the original size. A tiny economy but one you can effectively get for free.

There are other such economies such as grouping alike media queries but I'll leave these micro-optimisations for you to explore should they pique your interest.