I figured it out.
A couple things I failed to mention in the first place.
I created a Java Desktop Application in netBeans. This creates a JDesktop Application that starts with a main FrameView that has a MenuBar (JMneuBar), a mainPanel (JPanel), and a statusBar (JPanel). Inside the mainPanel, I added a JPanel with a couple labels and a JComboBox. Below that JPanel, I added the JScrollPane with the JTree inside that.
It was the first JPanel that was limiting the JScrollPanel width.
I deleted the first JPanle and added those components to the mainPanel witht the JScrollPane below them. It now resizes correctly.