Actually, in the default Metal LaF at least, it's just one Icon* which reacts to rollover/pressed/selected states by casting the Component passed to its paintIcon method to a JRadioButton and retrieving its model.
* private class javax.swing.plaf.metal.MetalIconFactory.RadioButtonIcon, obtainable via public static Icon getRadioButtonIcon()
Farhad, one easy way to get a larger circle could be to layer your own Icon, which simply draws a circle of the desired size, with the default Icon. See the HCenter/VCenter alignment sample on
http://tips4java.wordpress.com/2009/03/25/toggle-button-icons/