ItemsControl and GroupStyle issues

Question

The following code generates a UserControl for every item in a collection. The data is displayed correctly, but for some reasons which I can't understand, all the UserControls have the same size.

As you can see, Group2 contains multiple StackPanels. All these StackPanels have the very same dimensions and is not resizing according to the content.

Any idea why? Is it because of the StackPanel defined in the ItemsPanelTemplate?

            <CollectionViewSource x:Key="test" Source="{Binding}">
                <CollectionViewSource.GroupDescriptions>
                    <PropertyGroupDescription PropertyName="Column"/>
                </CollectionViewSource.GroupDescriptions>
            </CollectionViewSource> 

            <ItemsControl ItemsSource="{Binding Source={StaticResource test}}" HorizontalAlignment="Center">
                    <ItemsControl.ItemTemplate>
                        <DataTemplate>
                            <controls:Template/>
                        </DataTemplate>
                    </ItemsControl.ItemTemplate>
                    <ItemsControl.ItemsPanel>
                        <ItemsPanelTemplate>
                            <StackPanel Orientation="Vertical"/>
                        </ItemsPanelTemplate>
                    </ItemsControl.ItemsPanel>
                    <ItemsControl.GroupStyle>
                        <GroupStyle>
                            <GroupStyle.Panel>
                                <ItemsPanelTemplate>
                                    <StackPanel Orientation="Horizontal"/>
                                </ItemsPanelTemplate>
                            </GroupStyle.Panel>
                        </GroupStyle>
                    </ItemsControl.GroupStyle>
                </ItemsControl>