[Plugin: WP Smush.it] Size not reduced

Hi,

How to verify this plugin is working or not?
In the Media page, I can see there is Smush.it column says that my image “Reduced by 17.5% (29.9 KB)” for example. But when I download the image, its size is just same with original.

Thanks!

http://wordpress.org/extend/plugins/wp-smushit/