Why this the textbox will not have the maximum of 3 colums, as specified by `size`?

Why this the textbox will开发者_运维技巧 not have the maximum of 3 colums, as specified by size:

<?xml version="1.0"?>
<?xml-stylesheet href="chrome://global/skin/" type="text/css"?>

<window
    id="theWindow"
    title="The Window"
    orient="horizontal"
        width = "400"
        height = "300"
    xmlns="http://www.mozilla.org/keymaster/gatekeeper/there.is.only.xul"
    xmlns:html="http://www.w3.org/1999/xhtml">
    <textbox multiline="true" size="3" wrap="on"/>
</window>

A textbox needs additional horizontal space for the scrollbar that might become visible. So size="3" really only means that at least 3 characters can be displayed - but it could be more.