Java - Why is my Serializable object file size so large?

So I have an image, called square.png, that is about 3.7 kB in size. I read it into a BufferedImage as so:

BufferedImage img = ImageIO.read("square.png");

At this point I tried writing several different objects to the disk.

ObjectOutputStream stream = new ObjectOutputStream(new FileOutputStream("square_out.data"));

I tried the image wrapped in an ImageIcon: stream.writeObject(new ImageIcon(img));

I tried a 2D array of the same size as the pixel dimension of the image (800x600).

I tried wrapping that array in a custom class that implements Serializable and writing that.

All the above techniques resulted in a square_out.data with a size of about 1.9 MB. That's huge considering the original image was only a handful of kilobytes. It's also odd because the size was exactly the same for each. Is there any reasonable explanation for this/is there a way around it? I'd like to store lots of these, so the absurd file size is bothersome.