In Internet Explorer and Firefox/Mozilla, the default font size on buttons is "small". This holds for both <button> and <input type="button">.
Internet Explorer has a problem if absolute sizes are used to specify the font size in quirks mode. All the font sizes are one size too large. For example, if "small" is specified, in quirks mode the button text will be medium. The sizes are correct in standards mode.
Firefox/Mozilla has a problem if the font-family is specified. The default font size increases from small to medium.