why font CSS can not apply to QPushButton on hover?

I write the css for my demo application:

Code:

---

QPushButton
{
    font-size:8px;
    color:#333333;
    border: 1px solid red;
    background-color: green;
}
QPushButton:hover
{
    font:bold "Arial";
    font-size:20px;
    color:blue;
    border: 1px solid green;
    background-color: red;
}

---

but the font size never changed when hover on my buttons. what's the reason?:confused: