Bitmap.RotateFlip resizing image

Question

I'm iterating over images in a folder and rotating them using the RotateFlip method of the Bitmap class.  The images start out at around 6MB before rotation, but end up 1.32MB after.  Here's a snipet of the code I'm using:

private void ProcessFile(FileInfo fi)
      {
        Bitmap bmp = new Bitmap(fi.FullName);
        EXIFextractor exif = new EXIFextractor(ref bmp, "\n");
        if (exif["Orientation"] != null)
        {
            string orientation = exif["Orientation"].ToString();
            RotateFlipType rotateFlipType = OrientationToFlipType(orientation);
            if (rotateFlipType != RotateFlipType.RotateNoneFlipNone)
            {
                //rotate the image
                bmp.RotateFlip(rotateFlipType);
                if (chkUpdateOrientationProperty.Checked)
                {
                    try
                    {
                        //reset the orientation flag
                        exif.SetOrientationTag(1);
                    }
                    catch { }
                }
                //save the image
                string newFileName = (chkRotateOriginals.Checked) ? fi.FullName : SaveAsFileName(fi);
                bmp.Save(newFileName, ImageFormat.Jpeg);
            }
        }
        else
        {
        }
        bmp.Dispose();
    }

 

If I comment out the line 

bmp.RotateFlip( rotateFlipType );

the image size is not affected, so I've narrowed down the problem to that one line of code.  I've checked the properties of the original and rotated images and the dimensions aren't any smaller (just rotated), dpi, bit depth, etc are all the same.  So I'm at a loss as to why the rotated image's file size is so much smaller.  Does anyone have any idea what would cause this and if there is a way to prevent this from happening?  Thanks in advance.