Image size getting decreased after converting it into byte[] using BufferedImage and ImageIO

I am converting an Image into byte[] using following code.

public static byte[] extractBytes (String ImageName) throws IOException {

   ByteArrayOutputStream baos=new ByteArrayOutputStream();
        BufferedImage img=ImageIO.read(new File(ImageName));
        ImageIO.write(img, "jpg", baos);
        return baos.toByteArray();
    }

Now when I am testing my code:

public static void main(String[] args) throws IOException {
    String filepath = "image_old.jpg";
    File outp=new File(filepath);
    System.out.println("Size of original image="+outp.length());
    byte[] data = extractBytes(filepath);
    System.out.println("size of byte[] data="+data.length);
    BufferedImage img = ImageIO.read(new ByteArrayInputStream(data));
    //converting the byte[] array into image again
    File outputfile = new File("image_new.jpg");
    ImageIO.write(img, "jpeg", outputfile);
    System.out.println("size of converted image="+outputfile.length());
}

I am getting very strange results:

Size of original image=78620 
size of byte[] data=20280
size of converted image=20244

After converting image into byte[], its size getting decreased by around 1/4th and also when I am converting byte[] back to image its size alters.But output image is successfully getting created in the desired location.