Using a stylesheet to customize a QSlider

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Consider the following code snippet that creates a single QSlider:
@
MainWindow::MainWindow(QWidget *parent) :
QMainWindow(parent)
{
resize(700, 800);
setStyleSheet("QSlider { width: 40px }"
"QSlider::groove:horizontal {"
"border: 1px solid #999999;"
"background: qlineargradient(x1:0, y1:0, x2:0, y2:1, stop:0 #B1B1B1, stop:1 #c4c4c4);"
"height: 50px;"
"margin: 0 0 0 0;}"
"QSlider::handle:horizontal {"
"background: qlineargradient(x1:0, y1:0, x2:1, y2:1, stop:0 #b4b4b4, stop:1 #8f8f8f);"
"border: 1px solid #5c5c5c;"
"width: 50px;"
"margin: -5px 0 -5px 0;"
"border-radius: 3px;}");

QSlider *slider;
// Here are two ways of creating a qslider

// the following line of code makes a slider of about 120 pixels in width
// changing the first width & height in the stylesheet have no effect
// but changing the last width (handle width) does work

// slider = new QSlider(Qt::Horizontal, this);

// the following two lines of code make a slider that's much wider (around 650 pixels)
// changing the first width in the stylesheet has no effect
// but changing the groove height and handle width do take effect

// slider = new QSlider(Qt::Horizontal);
// slider->setParent(this);

slider->move(25, 100);

@
Ultimately, I'd like to make this QSlider vertical, but for right now, I'm wondering why the two ways of instantiating the slider behave so differently. I looked around for a good explanation of what's going on but couldn't find any.