Zoom a wmf image file?

here is my code

 public Image PictureBoxZoom(Image img, Size size)
        {
            //Bitmap bm = new Bitmap(img, Convert.ToInt32(img.Width * size.Width), Convert.ToInt32(img.Height * size.Height));
            Metafile imgOriginal = new Metafile("pic/Iran_political.wmf");
            Graphics grap = Graphics.FromImage(imgOriginal);
            grap.InterpolationMode = System.Drawing.Drawing2D.InterpolationMode.HighQualityBicubic;
            return imgOriginal;
        }

        private void zoomSlider_Scroll(object sender, EventArgs e)
        {
            if (zoomSlider.Value > 0)
            {
                pictureBox1.Image = null;
                pictureBox1.Image = PictureBoxZoom(imgOriginal, new Size(zoomSlider.Value, zoomSlider.Value));
            }
        }

on the bold line, it has an error : out of memory!

why?
how can i solve it?
my image size is about 5000 * 6000 , it's a map (wmf format picture).