[C#]DrawImage will make picture volume bigger

Question

var map = new Bitmap("a.jpg");
var w = map.Width;
var h = map.Height;
var _bitmap = new Bitmap(w, h, PixelFormat.Format24bppRgb);
Graphics _g = Graphics.FromImage(_bitmap);
var ret = new Rectangle(0, 0, w, map1.Height);
_g.DrawImage(map, ret);
_bitmap.Save("a.jpg");

I stitch the picture, 6000px*4000px, source image about 6mb, when my program run over,it bigger 10 times than before, over 60mb. I searched everywhere,but got nothing.

Looking forward your answer.

Answer 1

I forgot set jpg format

bitmap.Save("a.jpg", System.Drawing.Imaging.ImageFormat.Jpeg);

problem is solved.