How do I make the QSlider groove very thin using stylesheets?

[mayaknife]

I have the following in my stylesheet:

QSlider::groove:horizontal {
    border: 1px solid #999999;
    height: 8px;
    margin: 2px 0px;
}

but the groove comes out 15 pixels tall. It doesn't seem to matter what I set the height to the groove is always 15 pixels tall.

Is there a way to get a narrower groove using stylesheets?