I ran your program as you posted it, and noted
the same effect you report, file size increasing
from an initial 8Kb, to 9.5, then 11.

Looking in the ser file, I noted a lot of cruft gets
serialised as well, so decided to try a run just
serializing the data.*

The size dropped to around 290-310 bytes (including
4 smallish values in the table data), but was still a little
'indefinite'.

Why not use XMLEncoder/Decoder instead?
The advantages are twofold.
- Serialization might well fail between class changes
(read as J2SE upgrades) - it is fragile.
- The XMLEncoder is optimized to store only what is
necessary, and will probably lead to a more predictable
file size.

* In any case, here is my variant that focuses on the
table data itself..

<sscce>
/*
* NewJFrame.java
*
* Created on April 8, 2007, 11:40 PM
*/

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

public class NewJFrame extends javax.swing.JFrame {

   Vector columnIdents;

  /** 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();
      columnIdents = new Vector();
      for (int ii=1; ii<5; ii++) {
          columnIdents.addElement(
              new String("Title " + (ii)));
      }
      Vector data = new Vector();
      for (int ii=1; ii<5; ii++) {
           data.add( new Vector(4) );
      }

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(
          data, columnIdents
      ));
      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", false);
          ObjectOutputStream out = new ObjectOutputStream(fos);
          out.writeObject(((DefaultTableModel)jTable1.
              getModel()).getDataVector());
          out.close();
      }
      catch(IOException exp) {
          exp.printStackTrace();
          return false;
      }

      return true;
  }

  private boolean loadTableModel() {
      //TableModel tableModel = null;
      Vector data = null;

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

      //if(tableModel != null)
      if(data != null) {
          ((DefaultTableModel)jTable1.getModel()).
              setDataVector(data, columnIdents);
      }

      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

}
</sscce>

HTH

DefaultTableModel extends AbstractTableModel and this class serializes
all listeners that implement Serializable. Since JTable register itself
as a listener with its table model, you are actually serializing the
whole JTable instance along with the table model. When data is
deserialized back in, the old JTable instance will just "sit" inactive
in the listener list of the table model and next time you serialize the
table model you will now get 2 JTable instances written out. And so on.

The fittest work-around seems to be what Andrew did, namely to serialize
only the vector data of the table model.

Regards,