Use this before you start the program:
UIManager.put( "Button.margin", new InsetsUIResource(10, 20, 10, 20) );
BTW: To global change the icon of you JTree use:
Icon leafIcon = new ImageIcon( bla );
UIManager.put( "Tree.leafIcon", leafIcon );
UIManager.put( "Tree.openIcon", openIcon );
UIManager.put( "Tree.closedIcon", closedIcon );