Surface3D size is just 1x1

[Tom asso]

I'm writing a QML application with C++ singleton back-end.
My qml defines an Item which contains a Surface3D:

ApplicationWindow {
    visible: true
    width: 640
    height: 480
    title: qsTr("Hello World")

    Item {
        width: 640
        height: 480
        objectName: "surface3DItem"

        Surface3D {
	    objectName: "surface3D"
            width: parent.width
            height: parent.height

After my C++ is instantiated, my code reads the width and height of Item (named "surface3DItem") and width and height of Surface3D (named "surface3D"):

object = g_rootWindow->findChild<QObject *>("surface3DItem");
qDebug() << "surface3DItem width: " << ((QQuickItem *)object)->width();
qDebug() << "surface3DItem height: " << ((QQuickItem *)object)->height();

object = g_rootWindow->findChild<QObject *>("surface3D");
qDebug() << "surface3D width: " << ((Surface3D *)(object)->width();
qDebug() << "surface3D height: " << ((Surface3D *)(object)->height();

Since my qml explicitly specifies Item width=640 and height=480, I would expect the enclosed Surface3D to have the same. But instead my app prints:

surface3DItem width:  640
surface3DItem height:  480
surface3D width:  1
surface3D height:  1

Why is the Surface3D only 1x1, while its parent Item is 640x480?

Thanks
Tom