contentItem doesn't follow window size

#include <QtCore>
#include <QtQuick>

int main(int argc, char* argv[])
{
    QGuiApplication app(argc, argv);

    QQmlEngine engine;
    QQuickWindow window;
    window.setWidth(400);
    window.setHeight(400);
    QQmlComponent component(&engine);
    component.setData(QByteArray("import QtQuick 2.0\n Rectangle { color: 'red'; anchors.fill: parent }"), QUrl());
    QObject *object = component.create();
    qobject_cast<QQuickItem*>(object)->setParentItem(window.contentItem());
    qDebug() << object << window.contentItem()->width() << window.contentItem()->height();

    window.show();
    return app.exec();
}