For a QQuickPaintedItem if setContentsScale( 2.0 ) ( or setContentsSize() ) is set then the contents rendered in paint appears scaled in a transformed location. According to the documentation I would expect the render texture to be twice the size and painted at half the scale.

e.g.
setContentsScale( 2 )

void QQuickGraphicsView::paint( QPainter * painter )
{
painter->drawRect( 10, 10, 100, 200 );
}