Serialized Object size?

I am writing an object to a file. The object is basically a data chunk from a file, with some other information added. The thing I don't understand is that the input file is around 10 KB, but the output file is about 20 KB, but data I add one only a few bytes.

What could be the reason? Are there some object identifiers, attributes also logged and occupy some space apart from the data chunk.

Code Snippet:

public void writePacket(Packet packet) {
  String fileName = packet.getName();
  String filePath = dtnStore+fileName ;
  ObjectOutputStream out = null;
  File file = new File(filePath);
  if(!file.exists())
    out =  new ObjectOutputStream(new FileOutputStream   (filePath));
  else
    out = new AppendableObjectOutputStream (new FileOutputStream (filePath, true));
  out.writeObject(packe开发者_如何学运维t);
  out.flush ();
}

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ObjectOutputStream is a reasonable verbose format. It includes a lot of information about how to reconstruct an object including its types. Most file formats don't contain this information which makes them smaller but not as convenient for a Java developer to use.

Unless your input is from an ObjectInputStream the file size can take dramatically.

e.g. say you read an Integer from a text file say 100\n is 4 bytes, but when you writ this to a ObjectOutputSTream it will be 81 bytes long as it include a header, the type of the object, Number the field names etc.