Content Button does not fit at all!

hi,

XAML:

    <Button Content="+"
                            HorizontalContentAlignment="Center"
                            VerticalContentAlignment="Center" Grid.Column="3" FontWeight="Light"  Margin="5,193,86,-21" Grid.Row="7"
                            Width="20" Height="22" RenderTransformOrigin="2.85,0.155"/>

A picture is worth a thousand words:

You can see how cross symbol appears at bottom instead of appears at the center.. Regardless how you increase the size of the button, always appears far of the centre, why?

Primary platform is Windows 7 Ultimate 64 bit along with VS 2012/Sql2k8 for WPF/SilverLight projects.