Button not filling grid cell
===

I'm trying to build a simple 3x3 grid in Silverlight with a button in each grid cell.  The grid definition is below.  When I add button to the grid they never fill the 130x130 grid cell.  I set the margin and padding to 0 on the buttons as well as setting their horizontal and vertical alignment to Stretch.

        <Grid x:Name="Test" ShowGridLines="True" HorizontalAlignment="Center">
            <Grid.RowDefinitions>
                <RowDefinition Height="130"></RowDefinition>
                <RowDefinition Height="130"></RowDefinition>
                <RowDefinition Height="130"></RowDefinition>
                <RowDefinition Height="130"></RowDefinition>
            </Grid.RowDefinitions>
            <Grid.ColumnDefinitions>
                <ColumnDefinition Width="130"></ColumnDefinition>
                <ColumnDefinition Width="130"></ColumnDefinition>
                <ColumnDefinition Width="130"></ColumnDefinition>
            </Grid.ColumnDefinitions>
