asp.net 为啥生成的缩略图比源文件还要大

public class ImageThumbnailMake    {        /// <summary>        /// 生成缩略图        /// </summary>        /// <param name="originalImagePath">源图路径（相对路径）</param>        /// <param name="thumbnailPath">缩略图路径（相对路径）</param>        /// <param name="width">缩略图宽度</param>        /// <param name="height">缩略图高度</param>        /// <param name="mode">生成缩略图的方式</param>            public static void MakeThumbnail(string originalImagePath, string thumbnailPath, int width, int height, string mode)        {            System.Drawing.Image originalImage = System.Drawing.Image.FromFile(Utils.GetMapPath(originalImagePath));            int towidth = width;            int toheight = height;            int x = 0;            int y = 0;            int ow = originalImage.Width;            int oh = originalImage.Height;            switch (mode)            {                case "HW"://指定高宽缩放（可能变形）                                    break;                case "W"://指定宽，高按比例                                        toheight = originalImage.Height * width / originalImage.Width;                    break;                case "H"://指定高，宽按比例                    towidth = originalImage.Width * height / originalImage.Height;                    break;                case "Cut"://指定高宽裁减（不变形）                                    if ((double)originalImage.Width / (double)originalImage.Height > (double)towidth / (double)toheight)                    {                        oh = originalImage.Height;                        ow = originalImage.Height * towidth / toheight;                        y = 0;                        x = (originalImage.Width - ow) / 2;                    }                    else                    {                        ow = originalImage.Width;                        oh = originalImage.Width * height / towidth;                        x = 0;                        y = (originalImage.Height - oh) / 2;                    }                    break;                default:                    break;            }            //新建一个bmp图片            System.Drawing.Image bitmap = new System.Drawing.Bitmap(towidth, toheight);            //新建一个画板            System.Drawing.Graphics g = System.Drawing.Graphics.FromImage(bitmap);            //设置高质量插值法            g.InterpolationMode = System.Drawing.Drawing2D.InterpolationMode.High;            //设置高质量,低速度呈现平滑程度            g.SmoothingMode = System.Drawing.Drawing2D.SmoothingMode.HighQuality;            //清空画布并以透明背景色填充            g.Clear(System.Drawing.Color.Transparent);            //在指定位置并且按指定大小绘制原图片的指定部分            g.DrawImage(originalImage, new System.Drawing.Rectangle(0, 0, towidth, toheight),                new System.Drawing.Rectangle(x, y, ow, oh),                System.Drawing.GraphicsUnit.Pixel);            try            {                //保存缩略图                bitmap.Save(Utils.GetMapPath(thumbnailPath));            }            catch (System.Exception e)            {                throw e;            }            finally            {                originalImage.Dispose();                bitmap.Dispose();                g.Dispose();            }        }    }        /// <summary>        /// 获得当前绝对路径        /// </summary>        /// <param name="strPath">指定的路径</param>        /// <returns>绝对路径</returns>        public static string GetMapPath(string strPath)        {            string path = "";            if (HttpContext.Current != null)            {                return HttpContext.Current.Server.MapPath(strPath);            }            else //非web程序引用            {                strPath = strPath.Replace("/", "\\");                if (strPath.StartsWith("\\"))                {                    strPath = strPath.Substring(strPath.IndexOf('\\', 1)).TrimStart('\\');                }                path = System.IO.Path.Combine(AppDomain.CurrentDomain.BaseDirectory, strPath);                return path;            }        }