QGraphicsVideoItem as viewfinder in QCamera cannot be scaled to fullscreen size

I am using a QGraphicsVideoItem as the viewfinder in a QCamera in fullscreen landscape mode on Nokia N8. The size of the GraphicsItem is set to 640x480 and the aspect ratio mode to Qt::KeepAspectRatioByExpanding.

Expected: the viewfinder frames are displayed fullscreen (missing 120 pixel rows at the bottom).

Found: the viewfinder frames are always displayed in 480x360 resolution centered on the screen.

This also affects QDeclarativeCamera, setting 'width' and 'height' in qml does not have the expected effect.

code:
...
m_camera = new QCamera();
m_viewfinderItem = new QGraphicsVideoItem(this);
m_viewfinderItem->setSize(QSize(640, 480));
m_viewfinderItem->setAspectRatioMode(Qt::KeepAspectRatioByExpanding);
m_camera->setViewfinder(m_viewfinderItem);
...
m_camera->start()