Probably the fastest way to achieve this is to use the compressed option, mentioned in the previous comments, as an argument. The quickest way to configure this in PHPStorm is as follows:
- Go to
File > Settings
- Inside
Project Settings select File Watchers
- You should already have an SCSS watcher created here (if you have the SCSS watch plugin enabled, PHPStorm prompts you to create a watcher when opening a new .scss file.) Otherwise, enable it (more info about that in this section of the official documentation,) and then create the new watcher pressing the "+" symbol.
- Double click the watcher name to access its configuration.
- In the
Arguments line make sure to add the --style compressed argument
- Click OK and you're done
This image shows how that configuration should look:

From that point on, your .css output files will be compressed.
--style compressedoption, which will produce very compact/compressed output -- sass-lang.com/docs/yardoc/file.SASS_REFERENCE.html#output_style . If you wish -- you can then process it again with YUICompressor (to possibly obtain few more optimisations). Alternatively create a .BAT/.CMD file that would compile sass code first and then run it trough YUIC (2 lines in total) and use it in your File Watcher instead of actual SASS compiler.