StackView doesn't fit its content

import QtQuick 2.9
import QtQuick.Window 2.2
import QtQuick.Controls 2.4
import QtQuick.Layouts 1.3

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

    MouseArea {
        anchors.fill: parent
        onDoubleClicked: dialog.open()
    }

    Dialog {
        id: dialog
        title: "I'm dialog"
        contentItem: StackView {
            initialItem: Rectangle {
                width: 200; height: 200
                color: "red"
            }
        }
    }
}