上传图像后不减小asp.net中的大小...？

我的要求是上传图片并添加到水印图片，其工作正常，但主图像大小为1mb后添加水印相同的图像大小是400kb .size减少但我的要求大小正在增加我正在写这个代码.....在drawimage方法大小减少后请给我一个解决方案....

my requirement is upload the image and add to watermark image its working fine but the main image size is 1mb after added the watermark in same image size is 400kb .size is reducing but my requirement size is increasing i am writing this code.....after drawimage method size is reducing please give me a solution....

protected void Button1_Click(object sender, EventArgs e)
   {
       FileName = Guid.NewGuid().ToString();
       FilePath = @"C:\Users\bravemountravi\Desktop\Image\Attachments\" + FileName + "." + "jpg";

       System.Drawing.Image Mainimg = System.Drawing.Image.FromFile(@"C:\Users\travi\Desktop\Image\Attachments\funny-dog-wallpaper-and-picture-p0d-funny-dog-photo-hd-ahhw.jpg");

           System.Drawing.Image img2 = System.Drawing.Image.FromFile(@"C:\Users\travi\Desktop\Image\Images\watermark3.png");

           Graphics g = Graphics.FromImage(Mainimg);



           double _scalefactor = (Mainimg.Width / 4800.0);

           if (_scalefactor <= 0.1)
           {
               _scalefactor = 0.1;
           }
           else    if (_scalefactor <= 0.2)
           {
              _scalefactor = 0.2;
           }

           else if (_scalefactor >= 1)
           {
               _scalefactor = 1.0;
           }


           double _newwidth = (_scalefactor * img2.Width);
           //double _newHeight = ((_newwidth / img2.Width) * (img2.Height));

           double _newHeight = (_scalefactor * img2.Height);


           Bitmap bitimg = WaterMarkScaleImage(img2, Convert.ToInt32(Math.Round(_newwidth, 0)), Convert.ToInt32(Math.Round(_newHeight, 0)));
           System.Drawing.Image scaleimg = (System.Drawing.Image)bitimg;


           int left = Mainimg.Width - scaleimg.Width;
           int top = Mainimg.Height - scaleimg.Height;

         g.DrawImage(scaleimg, left, top, scaleimg.Width, scaleimg.Height);

           g.Dispose();
          Mainimg.Save(FilePath);

   }
   public Bitmap WaterMarkScaleImage(System.Drawing.Image image, int maxWidth, int maxHeight)
   {
       var newImage = new Bitmap(maxWidth, maxHeight);
       Graphics.FromImage(newImage).DrawImage(image, 0, 0, maxWidth, maxHeight);
       Bitmap bmp = new Bitmap(image);
       return bmp;
   }