Serialization file getting bigger and bigger on the same object

In my application, I perform serialization on my JTable's TableModel.
I realize that, even I didn't make any modification (either save or
delete data) on the JTable content, however, everytime I save the
JTable's TableModel, the serialization file will getting bigger and
bigger (about 2KB)

It works this way

1. I shut down my application. My application save JTable's TableModel
to serialization file. The file size is 50 KB

2. I open up my application. My application load JTable's TableModel.
Everything in the JTable content restore.

3. I shut down my application again, without modification on the
JTable's TableModel. The file size is 52 KB.

4. If I keep repeating step 1, the file size will keep increase.

May I know how I can prevent my serialization file from getting bigger
and bigger?

Thanks

--------code----------
/*
 * NewJFrame.java
 *
 * Created on April 8, 2007, 11:40 PM
 */

import java.io.*;
import javax.swing.table.*;

/**
 *
 * @author doraemon
 */
public class NewJFrame extends javax.swing.JFrame {

    /** Creates new form NewJFrame */
    public NewJFrame() {
        initComponents();

        loadTableModel();
    }

    /** This method is called from within the constructor to
     * initialize the form.
     * WARNING: Do NOT modify this code. The content of this method is
     * always regenerated by the Form Editor.
     */
    // <editor-fold defaultstate="collapsed" desc=" Generated Code
">
    private void initComponents() {
        jScrollPane1 = new javax.swing.JScrollPane();
        jTable1 = new javax.swing.JTable();

setDefaultCloseOperation(javax.swing.WindowConstants.DISPOSE_ON_CLOSE);
        addWindowListener(new java.awt.event.WindowAdapter() {
            public void windowClosed(java.awt.event.WindowEvent evt) {
                formWindowClosed(evt);
            }
        });

        jTable1.setModel(new javax.swing.table.DefaultTableModel(
            new Object [][] {
                {null, null, null, null},
                {null, null, null, null},
                {null, null, null, null},
                {null, null, null, null}
            },
            new String [] {
                "Title 1", "Title 2", "Title 3", "Title 4"
            }
        ));
        jScrollPane1.setViewportView(jTable1);

        javax.swing.GroupLayout layout = new
javax.swing.GroupLayout(getContentPane());
        getContentPane().setLayout(layout);
        layout.setHorizontalGroup(

layout.createParallelGroup(javax.swing.GroupLayout.Alignment.LEADING)
            .addGroup(layout.createSequentialGroup()
                .addContainerGap()
                .addComponent(jScrollPane1,
javax.swing.GroupLayout.PREFERRED_SIZE, 375,
javax.swing.GroupLayout.PREFERRED_SIZE)
                .addContainerGap(15, Short.MAX_VALUE))
        );
        layout.setVerticalGroup(

layout.createParallelGroup(javax.swing.GroupLayout.Alignment.LEADING)
            .addGroup(layout.createSequentialGroup()
                .addComponent(jScrollPane1,
javax.swing.GroupLayout.PREFERRED_SIZE, 275,
javax.swing.GroupLayout.PREFERRED_SIZE)
                .addContainerGap(25, Short.MAX_VALUE))
        );
        pack();
    }// </editor-fold>

    private void formWindowClosed(java.awt.event.WindowEvent evt)
{
// TODO add your handling code here:
        saveTableModel();
        System.out.println("table model saved");
    }

    private boolean saveTableModel() {
         try {
            FileOutputStream fos = new
            FileOutputStream("table.ser");
            ObjectOutputStream out = new ObjectOutputStream(fos);
            out.writeObject(jTable1.getModel());
            out.close();
        }
        catch(IOException exp) {
            exp.printStackTrace();
            return false;
        }

        return true;
    }

    private boolean loadTableModel() {
        TableModel tableModel = null;

        try {
            FileInputStream fos = new FileInputStream("table.ser");
            ObjectInputStream in = new ObjectInputStream(fos);
            tableModel = (TableModel)in.readObject();
            in.close();
        }
        catch(IOException exp) {
            exp.printStackTrace();
            return false;
        }
        catch(ClassNotFoundException exp) {
            exp.printStackTrace();
            return false;
        }

        if(tableModel != null)
            jTable1.setModel(tableModel);

        return true;
    }

    /**
     * @param args the command line arguments
     */
    public static void main(String args[]) {
        java.awt.EventQueue.invokeLater(new Runnable() {
            public void run() {
                new NewJFrame().setVisible(true);
            }
        });
    }

    // Variables declaration - do not modify
    private javax.swing.JScrollPane jScrollPane1;
    private javax.swing.JTable jTable1;
    // End of variables declaration

}