Why select some text in a output window pane causes memory exhaustion?

I am encountered a problem that writing heavy data endlessly to a output window pane caused "Out of memory" exception of Visual studio 2012 (update 3 & 4). I am using OutputWindowPane::OutputString() to write data.

It seems there is an algorithm to limit the text of output window and it does work. But I found there might be an exception as described below:

After writing heavy data (e.g. 1M chars per line) with a time interval of 100mS for a long time (say >10 minutes), the memory(private working set) of devenv process will be stable at about several MegaBytes. Here, if I select some text shown in the output window pane by mouse then click Ctrl+C, the memory of devenv process will increase sharply and in some cases "Out of memory" exception will be raised. This problem has been watched in all my two computers with a high reproducing rate (>50%). Why selecting text causing such big difference? Is there something I should be careful with the text document associated with output window pane?

My reproduce program is described below:

    class Program
    {
        private static EnvDTE.DTE _dte = null;
        private static EnvDTE80.DTE2 _dte2 = null;

        private static EnvDTE.OutputWindow _outputWindow = null;
        private static OutputWindowPane _sitePane = null;
        private static bool _stopFlag = false;

        static void Main(string[] args)
        {
            _dte = (EnvDTE.DTE)Marshal.GetActiveObject(@"VisualStudio.DTE.11.0");
            _dte2 = _dte as EnvDTE80.DTE2;
            _outputWindow = _dte2.ToolWindows.OutputWindow;
            string paneName = "Site1";

            try
            {
                _sitePane = _outputWindow.OutputWindowPanes.Item(paneName);
            }
            catch (System.Exception)
            {
                _sitePane = _outputWindow.OutputWindowPanes.Add(paneName);
            }

            System.Threading.Thread thread = new System.Threading.Thread(WriteOutputPane);
            thread.Start();

            Console.WriteLine("Press a key to finish");
            Console.ReadLine();
            _stopFlag = true;

            thread.Join();
            Console.WriteLine("Done...");
        }


        static protected void WriteOutputPane()
        {
            Console.WriteLine("thread started");
            string digit = @"0123456789012345"; //16 chars
            string msg = "";
            const int MsgLength = 1024*1024;
            while(msg.Length < MsgLength )
            {
                msg += digit;
            }

            int loopIndex = 0;
            while (_stopFlag == false)
            {
                try
                {
                    string formattedLoop = string.Format("{0, 8:D4}::::::::", loopIndex++);
                    string finalMsg = (formattedLoop + msg).Remove(MsgLength - 1) + Environment.NewLine;

                    int OuputCnt = 1;
                    for (int cnt = 0; cnt < OuputCnt; ++cnt)
                    {
                        _sitePane.Activate();
                        _sitePane.OutputString(finalMsg);
                        //_sitePane.ForceItemsToTaskList();
                    }
                    System.Threading.Thread.Sleep(100);
                }
                catch (Exception ex)
                {
                    Console.WriteLine("-- ignore exception -- ");
                    Console.WriteLine("   {0}", ex.Message);
                }
            }
        }
    }
}