Java SE (Java Platform, Standard Edition)

I have a strange behaviour with a basic JTabbedPane: each time I add a new tab, the JtabbedPane is growing even if each component is the same size. I was expecting that the JTabbedPanel will be the size of the biggest component when inside a BorderLayout.
Try this code:

import java.awt.*;
import java.awt.event.*;
import javax.swing.*;

public class Main
{

    public static void main(String args[])
    {
        Runnable runner = new Runnable()
        {

            public void run()
            {
                final JFrame frame = new JFrame("Tabbed Pane Sample");
                JPanel panel = new JPanel(new BorderLayout());
                frame.add(panel);
                frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
                final JTabbedPane tabbedPane = new JTabbedPane();
                panel.add(tabbedPane, BorderLayout.NORTH);
                JButton add = new JButton("Add");
                add.addActionListener(new ActionListener()
                {

                    public void actionPerformed(ActionEvent e)
                    {
                        tabbedPane.addTab("New tab", new JLabel("Same label"));
                    }
                });
                panel.add(add, BorderLayout.SOUTH);
                JTextArea text = new JTextArea("Alea jacta est, press the buttom 6 times if you can");
                panel.add(text, BorderLayout.CENTER);
                frame.setSize(400, 150);
                frame.setVisible(true);
            }
        };
        EventQueue.invokeLater(runner);
    }
}