dialog multi anglet

I'm trying to make a multi-tab dialog box when I click on a button every other button decreases in size except the one clicé   here is the code

public class Multi_Chat : MonoBehaviour {

 public static bool showChat = false;//vraible d'affiche de boite de chat
 private Vector2 scrollPosition ;//définition de barre de déroulement
 private Vector2 scrollPosition1 ;//définition de barre de déroulement
 private Rect window = new Rect(50, 50, 200, 300);//fêntre de chat
 private string chat             = "";
 private string message         = "";
 string[] values = new string[] { "Sun", "Mon", "Tue", "Wed", "Thu", "Fri", "Sat" };
 int a=100;
 int b=40;

void OnGUI () {
window = GUI.Window (5, new Rect(50, 50, 500, 400), GlobalChatWindow, "Chat avec "+ShowMessage.destinataire_name);
}

void GlobalChatWindow (int id) {
scrollPosition1 = GUILayout.BeginScrollView(scrollPosition1,true,false,GUILayout.Width(600),GUILayout.Height(70));

     for(int n = 0; n < values.Length; n++)
     {
        int y=88*n;          
                if (GUI.Button(new Rect(y+10*n, 10, a, b),values[n]))
            {
        }
         
     }    
 GUILayout.EndScrollView ();
 scrollPosition = GUILayout.BeginScrollView (scrollPosition);
 GUILayout.Label(chat);
 GUILayout.EndScrollView ();
 GUI.SetNextControlName("message");
 message = GUILayout.TextField(message);
 
 GUI.DragWindow();

}