Como não deixar um panel aumentar seu tamanho?

package teste;
import java.awt.GridLayout;
import javax.swing.*;

public class panel_tabuleiro extends JPanel {
       public panel_tabuleiro() {
        this.setSize(320, 300);
        this.setLayout(new GridLayout(10, 10, 1, 1));

        Icon agua = new ImageIcon(getClass().getResource("agua.gif"));
        JButton jButton[] = {new JButton("", agua), new JButton("", agua), new JButton("", agua), new JButton("", agua), new JButton("", agua), new JButton("", agua), new JButton("", agua), new JButton("", agua), new JButton("", agua), new JButton("", agua), new JButton("", agua), new JButton("", agua), new JButton("", agua), new JButton("", agua), new JButton("", agua), new JButton("", agua), new JButton("", agua), new JButton("", agua), new JButton("", agua), new JButton("", agua), new JButton("", agua), new JButton("", agua), new JButton("", agua), new JButton("", agua), new JButton("", agua), new JButton("", agua), new JButton("", agua), new JButton("", agua), new JButton("", agua), new JButton("", agua), new JButton("", agua), new JButton("", agua), new JButton("", agua), new JButton("", agua), new JButton("", agua), new JButton("", agua), new JButton("", agua), new JButton("", agua), new JButton("", agua), new JButton("", agua), new JButton("", agua), new JButton("", agua), new JButton("", agua), new JButton("", agua), new JButton("", agua), new JButton("", agua), new JButton("", agua), new JButton("", agua), new JButton("", agua), new JButton("", agua), new JButton("", agua), new JButton("", agua), new JButton("", agua), new JButton("", agua), new JButton("", agua), new JButton("", agua), new JButton("", agua), new JButton("", agua), new JButton("", agua), new JButton("", agua), new JButton("", agua), new JButton("", agua), new JButton("", agua), new JButton("", agua), new JButton("", agua), new JButton("", agua), new JButton("", agua), new JButton("", agua), new JButton("", agua), new JButton("", agua), new JButton("", agua), new JButton("", agua), new JButton("", agua), new JButton("", agua), new JButton("", agua), new JButton("", agua), new JButton("", agua), new JButton("", agua), new JButton("", agua), new JButton("", agua), new JButton("", agua), new JButton("", agua), new JButton("", agua), new JButton("", agua), new JButton("", agua), new JButton("", agua), new JButton("", agua), new JButton("", agua), new JButton("", agua), new JButton("", agua), new JButton("", agua), new JButton("", agua), new JButton("", agua), new JButton("", agua), new JButton("", agua), new JButton("", agua), new JButton("", agua), new JButton("", agua), new JButton("", agua), new JButton("", agua)};

        for (int x = 0; x < 100; x++) {
            this.add(jButton[x]);
        }
    }      
}

package teste;

import java.awt.BorderLayout;
import javax.swing.JFrame;

public class tela_principal extends JFrame {
    public panel_tabuleiro tab1;
    public panel_tabuleiro tab2;

    public tela_principal() {
        super("Batalha Naval");
        getContentPane().setLayout(new BorderLayout());

        tab1 = new panel_tabuleiro();
        tab2 = new panel_tabuleiro();

        this.setSize(600,700);
        this.setResizable(false);
        getContentPane().add(tab1,BorderLayout.WEST);
        getContentPane().add(tab2,BorderLayout.EAST);

        tab1.setPreferredSize(new java.awt.Dimension(300,300));
        tab2.setPreferredSize(new java.awt.Dimension(300,300));

    }

    public static void main(String[] arg) {
        tela_principal usan = new tela_principal();
        usan.setVisible(true);
    }
}