A CSS optimiser may be enabled by an admin in Settings > Site administration > Development > Experimental > Experimental settings. The tool analyses and refactors CSS before caching it.
When enabled CSS will be run through an optimisation process before being cached. The optimiser processes the CSS removing duplicate rules and styles, as well as white space removable and reformatting. Please note turning this on at the same time as theme designer mode is awful for performance but will help theme designers create optimised CSS.