Directx.Capture / Frame returned by CaptureInfo.CaptureFrame() length too big

Using Directx.Capture; to Capture a picture from a Webcam (Capture the Frame from the Preview PictureBox). Displaying Length of that Frame is sometimes (rare) ok : around 25000 Most of the time is more than 10x larger, around 320000! Why is that? width and height always the same in both cases though!

   private void startCap() {
        public Filter VideoInput = null;
        public Capture CaptureInfo = null;
        public Filters InputOptions = new Filters();
        this.VideoInput = this.InputOptions.VideoInputDevices[camNum];
        this.CaptureInfo = new Capture(this.VideoInput, null);
        this.CaptureInfo.PreviewWindow = pictureBox1;
        this.CaptureInfo.RenderPreview();
        this.CaptureInfo.FrameCaptureComplete += new Capture.FrameCapHandler(CaptureInfo_FrameCaptureComplete);
        CaptureInfo.CaptureFrame();
  }

   void CaptureInfo_FrameCaptureComplete(PictureBox Frame)
        {
             Image nwImage = Frame.Image;
             using (MemoryStream ms = new MemoryStream())
             {
                 nwImage.Save(ms, ImageFormat.Png);
                 Debug.WriteLine("length "+ms.Length);  //<===== length
                 ms.Close();
                 ms.Dispose();
             }
    }

Just saying, the ligter image with low length of 25000, is correct and displaying properly. And i would like to find where the problem comes from to keep the Frames ALWAYS around that size only. please help :)