There can be some byte-saving "magic" like value optimizations, e.g. color: #000 instead of color: #000000, border: 2px instead of border: 2px 2px 2px 2px, outline: 0 instead of outline: none, etc. Depends on how you write your CSS. Some minifiers can even group certain rules together if they share declarations, but that can be dangerous.
Oh, and ... don't be surprised if Page Speed continues to recommend minifying css/js even after you have minified them. You could compress your jpgs down to 10% (is it a blazing log? is it an orangutan? what's that greenish blob in the corner?) and they'd still recommend compressing images.