why font CSS can not apply to QPushButton on hover?

[homerli]

I write the css for my demo application:
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: