DataGrid WPF column size

Hi,

I have DataGrid in WPF and I set columns size in XAML. 

<Viewbox  Grid.Row="3" Grid.Column="0" Grid.ColumnSpan="4" VerticalAlignment="Top"><DataGrid Name="tabulka" Margin="10"><DataGrid.Columns><DataGridTextColumn Header="PB"    Width="100" /><DataGridTextColumn Header="A" Width="100"/><DataGridTextColumn Header="Kusů" /><DataGridTextColumn Header="Vloženo" /></DataGrid.Columns></DataGrid></Viewbox>

Items source for DataGrid is List<>. It looks like this:

class Bedna{ //list<> declaration for items
    public string PB { get; set; }
    public string A { get; set; }
    public int Kusu { get; set; }
    public int Vlozeno { get; set; }
}


List<Bedna> bedny = new List<Bedna>(); //create list

bedny.Add(new Bedna() { //add items to list
                PB = "PB test",
                A = "A test",
                Kusu = 5,
                Vlozeno = 2
            });

tabulka.ItemsSource = bedny; //and display them

But when I run it, instead of using the existing columns with setted width, it created some new ones with width as small as possible.  Cannot post images or links, so http : // goo . gl / nDnhPS

How to make it to use existing columns iinstead of creating new ones?

Thanks