Once you transfer the image to the Canvas, your image assumes the dimensions of the Canvas. You have to handle that. Tim has some recommendations Basic Canvas question that might help you.
If you want the identical image, you must set the saved image using the identical dimensions (H x W) in pixels. At the moment, you are not doing that. You are probably setting Canvas dimensions as Automatic as a guess. Be aware, you need to know the original image dimensions to do this.
Regards,
Steve