Hi SwinGirl,
Have you taken a look at the JSplitPane portion of the Swing Tutorial over on Sun's site? It goes into the details of the different properties of the JSplitPane that you need to set to get the desired behavior. Just off the top of my head, I think you need set the minimum sizes for the components in your split pane, set the resize weight, and set the split location. (Just setting the split location isn't enough, by itself.)
Here is a link that presents what I'm talking about in more detail.
HTH