I'm guessing you are adding the scrollpane to a panel and then adding the panel to the tabbed pane. This won't work because the default layout of a panel is a FlowLayout which respects the preferred size of the components added to it.
Either:
1) get rid of the panel
2) set the layout of the panel to a BorderLayout.
If you need more help (and for future questions) post a proper SSCCE that demonstrates the problem.