图片上传后，保存图片大小大了好几倍，由30k变成了200k，请问大侠们谁有解决方法啊？

using (System.Drawing.Image originalImg = System.Drawing.Image.FromFile(pPath))
             {
                 if (originalImg.Width == imageWidth && originalImg.Height == imageHeight)
                 {
                     //return SaveCutPic(pPath, pSavedPath, pPartStartPointX, pPartStartPointY, pPartWidth, pPartHeight,
                     //        pOrigStartPointX, pOrigStartPointY);
                      
                 }
                 string filename = DateTime.Now.ToString("yyyyMMddHHmmss") + ".jpg";
                 //string filePath = pSavedPath + "\\" + filename;
                 //pSavedPath = pSavedPath + "newsimages\\" + DateTime.Now.ToString("yyyy_MM_dd");                  
                 string curpath = pSavedPath  + strPath;
                 if (isJZ)
                 {
                     pSavedPath = curpath + DateTime.Now.ToString("yyMMdd") + "\\HTImages\\" + "200_246\\";
                 }
                 else
                 {
                     pSavedPath = curpath + DateTime.Now.ToString("yyMMdd") + "\\HTImages\\" + "310_224\\";
                 }
                 string filePath = pSavedPath +"\\"+filename;
                 if (!System.IO.Directory.Exists(pSavedPath))
                 {
                     Directory.SetCurrentDirectory(curpath);
                     if (isJZ)
                     {
                         System.IO.Directory.CreateDirectory(DateTime.Now.ToString("yyMMdd") + "\\HTImages\\" + "200_246\\");
                     }
                     else
                     {
                         System.IO.Directory.CreateDirectory(DateTime.Now.ToString("yyMMdd") + "\\HTImages\\" + "310_224\\");
                     }
                 }
                 Bitmap thumimg =MakeThumbnail(originalImg, imageWidth, imageHeight);
               
                 Bitmap partImg = new Bitmap(pPartWidth, pPartHeight);                 Graphics graphics = Graphics.FromImage(partImg);
                 Rectangle destRect = new Rectangle(new Point(pPartStartPointX, pPartStartPointY), new Size(pPartWidth, pPartHeight));//目标位置
                 Rectangle origRect = new Rectangle(new Point(pOrigStartPointX, pOrigStartPointY), new Size(pPartWidth, pPartHeight));//原图位置（默认从原图中截取的图片大小等于目标图片的大小）
                
                 ///文字水印  
                 Graphics G = Graphics.FromImage(partImg);
                 //Font f = new Font("Lucida Grande", 6);
                 //Brush b = new SolidBrush(Color.Gray);
                 G.Clear(Color.White);
                 // 指定高质量的双三次插值法。执行预筛选以确保高质量的收缩。此模式可产生质量最高的转换图像。 
                 G.InterpolationMode = InterpolationMode.HighQualityBicubic; 
                // 指定高质量、低速度呈现。 
                 G.SmoothingMode = SmoothingMode.HighQuality;
                 graphics.DrawImage(thumimg, destRect, origRect, GraphicsUnit.Pixel);                 //G.DrawString("Xuanye", f, b, 0, 0);
                 G.Dispose();                 originalImg.Dispose();
                 if (File.Exists(filePath))
                 {
                     File.SetAttributes(filePath, FileAttributes.Normal);
                     File.Delete(filePath);
                 }
                 partImg.Save(filePath);
                 
                 partImg.Dispose();
                 thumimg.Dispose();
这是按钮的保存方法，多谢大家不吝赐教！