If you inspect the jQuery UI website using your browser's web inspector, you can see that in the <iframe>
containing the buttons you mention, the following CSS is applied:
body {
font-family: "Trebuchet MS", "Helvetica", "Arial", "Verdana", "sans-serif";
/* This is making the text smaller */
font-size: 62.5%;
}
In other words, they modified the CSS font-size property to make the text display smaller.