Convert image into array of pixels, recreate image using pixels - size gets reduced. Why?

Question

I converted an image into an array of pixels. I saved RGB values in three seperate arrays.

Then, I tried creating the image using the same values (no manipulation). Original image was of 205kB and the new image is of 121kB in case of B&W image, and from 215kB to 96kB in case of colored image. Also, there is slight change in brightness (brightness gets increased and so does overall contrast). What is causing this?

I have tried both with colored and B&W images. Result was same.

Also, I ran the same code on the previous output image (96kB), the new output was still 96kB.

Codes-

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1) To read image:

int width = img.getWidth(null);
int height = img.getHeight(null);

pixelR = new int[width * height];
pixelG = new int[width * height];
pixelB = new int[width * height];

int index=0;

int r, g, b, gray, rgb;

for(int i=0; i<width; i++) {
  for (int j=0; j<height; j++) {
    rgb = img.getRGB(i, j);

    r = (rgb >> 16) & 0xFF;
    g = (rgb >> 8) & 0xFF;
    b = (rgb & 0xFF);

    pixelR[index]=r;
    pixelG[index]=g;
    pixelB[index]=b;
    index++;
  }
}

2) Write image:

     BufferedImage image = new BufferedImage(width, height, BufferedImage.TYPE_INT_RGB);

  int index=0;
  int val;
  int r, g, b;

  for (int i=0; i<width; i++) {
    for (int j=0; j<height; j++) {

      r=pixelR[index];
      g=pixelG[index];
      b=pixelB[index++];

      Color newColor = new Color((int)r, (int)g, (int)b);     
      image.setRGB(i, j, newColor.getRGB());
    }
  }

  File f = null;

  try{
    f = new File("<filepath>.jpg");
      ImageIO.write(image, "jpg", f);
    }catch(IOException e){
      System.out.println("Error: "+e);
    }
  }

Answer 1

You can try setting the compression level to different values and see how the file size differs depending on that. See Setting jpg compression level with ImageIO in Java for how to do that.

Doesn't explain the change in brightness and contrast though.