Why the Canvas size is set to 0 (zero) after making it a children of a StackPanel defined in XAML?

Question

Why the Canvas size is set to 0 (zero) after making it a children of a StackPanel defined in XAML?

1.        Test1
   1.        Create a canvas, set its size and then add it to the StackPanelDefinedInXAML.
   2.       After making it a child of the StackPanelDefinedInXAML, canvas.ActualWidth=0 and canvas.ActualHeight=0;
2.        Test2
   1.        Create a canvas, set its size and then add it to a StackPanelDefinedInCode
   2.       After making it a child of the StackPanelDefinedInCode, canvas.ActualWidth=5000 and canvas.ActualHeight=5000;

Why Test1 resets the size of the canvas and Test2 doesn’t reset the size of the canvas?

    <ScrollViewer Name="Scroller">
        <StackPanel Name="PanelDefinedInXAML" />
    </ScrollViewer>

    public sealed partial class MainPage : Page
    {
        public MainPage()
        {
            this.InitializeComponent();
        }

        protected override void OnNavigatedTo(NavigationEventArgs e)
        {
            FirstPage.Loaded += FirstPage_Loaded;
        }

        private void FirstPage_Loaded(object sender, RoutedEventArgs e)
        {
            Test1();
            Test2();
        }

        private void Test1()
        {
            Canvas canvas = new Canvas();
            canvas.Width = 5000;
            canvas.Height = 5000;

            PanelDefinedInXAML.Children.Add(canvas);
        }

        private void Test2()
        {
            Canvas canvas = new Canvas();
            canvas.Width = 5000;
            canvas.Height = 5000;

            StackPanel panelDefinedInCode = new StackPanel();
            Scroller.Content = panelDefinedInCode;

            panelDefinedInCode.Children.Add(canvas);
        }