You have to specify the target size. The overload you chose scales the image from the source dpi to the target dpi. As explained in another question, you should do this:
System.Drawing.Image oImage = new Bitmap(600, 450);
Graphics oGraphic = Graphics.FromImage(oImage);
oGraphic.FillRectangle(Brushes.White, 0, 0, 600, 450);
oGraphic.DrawImage(image, 0,0, image.Width, image.Height);
return (Bitmap)oImage;