Popup Windows phone 8

I create a popup in my windows phone 8 application but my popup take only 1/2 of the width screen, why ?

Xmal of my popup :

<UserControl x:Class="PhoneApp1.PopupRename"xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"xmlns:d="http://schemas.microsoft.com/expression/blend/2008"xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"mc:Ignorable="d"FontFamily="{StaticResource PhoneFontFamilyNormal}"FontSize="{StaticResource PhoneFontSizeNormal}"Foreground="{StaticResource PhoneForegroundBrush}" d:DesignWidth="480" d:DesignHeight="170"><Grid x:Name="LayoutRoot" Background="{StaticResource PhoneChromeBrush}"><Grid.RowDefinitions><RowDefinition Height="80"/><RowDefinition Height="*"/></Grid.RowDefinitions><Grid.ColumnDefinitions><ColumnDefinition Width="*"/><ColumnDefinition Width="*"/></Grid.ColumnDefinitions><Button x:Name="CancelButton" Content="Cancel" Margin="0,0,0,0" VerticalAlignment="Top" Grid.Row="1" Grid.Column="0"/><Button x:Name="ValideButton" Content="Valider" Margin="0,0,0,0" VerticalAlignment="Top" Grid.Row="1" Grid.Column="1"/><TextBox x:Name="ValueTextBox"  Margin="0,5,0,5" TextWrapping="Wrap" Text="" Grid.Row="0" Grid.Column="0" Grid.ColumnSpan="2" /></Grid>

And i use this c# code :

 Popup renamePopup = new Popup();            renamePopup.VerticalOffset = 100;            renamePopup.HorizontalOffset = 10;            PopupRename popupRename = new PopupRename();            renamePopup.Child = popupRename;            renamePopup.IsOpen = true;            popupRename.ValideButton.Click += (s, args) =>            {                renamePopup.IsOpen = false;                //this.text.Text = Mycontrol.tbx.Text;            };            popupRename.CancelButton.Click += (s, args) => { renamePopup.IsOpen = false; };