Swing sizing woes

Question

A JList in a scrollpane, placed in "Center" in a BorderLayout is taking only about half of the available space, obscuring much of the JList. I want to have the scrollpane fit to the available size, which isn't even something I've had to think about before.
This has baffled me for some hours now, and I've tried forcing it to size by using setSize() for the scrollpane as well as the JPanel it sits in. I've also tried every variation of revalidate/pack/doLayout/repaint for the JFrame, but I still get a tiny Jscrollpane.
My previous version of this drew itself fine but I have to change the code to allow for better data handling.
Please help if you see what I am missing.class MainFrame extends JFrame implements Constants
  //instance variables
  Dimension viewport = new Dimension(840,160);
  public static final int WIDTH = 850;
  public static final int HEIGHT = 300;
  MainFrame(TheTableModel model)
   //stuff
    setSize(WIDTH,HEIGHT);
    cp = this.getContentPane();
    cp.setLayout(new BorderLayout());
    northP = new JPanel();
    cp.add(northP,"North");
    southP = new JPanel();
    cp.add(southP,"South");
    tp = new TablePanel(model);
    tp.setSize(viewport);
    cp.add(tp,"Center");
    //some buttons
    //pack();
    //validate();
    //doLayout();
    setVisible(true);

Answer 1

Here's something odd:
when I replacecp.add(tp,"Center"); withcp.add(new JButton("this")); the button takes up all the space as expected. BUT, when I convert the JPanel tp to a JButton, like so:TablePanel(TheTableModel tm)
   allRecs = tm.allRecs;
    this.tm = tm;
    jl = new JList(tm);
    renderer = new TabListCellRenderer();
    renderer.setTabs(new int[] {40,140,240,340,440,540,640,740});
    jl.setCellRenderer(renderer);
    jl.setSelectionMode(
              ListSelectionModel.SINGLE_SELECTION);
    jl.addListSelectionListener(new
      ListSelectionListener()
        public void valueChanged(ListSelectionEvent e)
          Object select;//=(String)"objectString";
          if(!e.getValueIsAdjusting())
            select =jl.getSelectedValue();
    JScrollPane jsp = new JScrollPane(jl);
    add(new JButton("that"));
  } I get a small Jbutton when I add the tp object to the contentPane, as in my original post, with cp.add(tp,"Center").
Is that odd, or what? Why are the buttons different sizes, and how can I make the JPanel contents take up the appropriate space in the BorderLayout?