Between the left and right panels is the Split bar. If you want to see what the Split bar looks like, see the screenshot and description in the chapter Main Window Components. You can change the position of the Split bar to enlarge one panel at the expense of the other panel.
You can change the position of the Split bar by dragging it with the mouse. To begin, press and hold the left mouse button on the Split bar. The Split bar also has a right-click context menu with percentage proportions for the left and right panel widths (20/80, ..., 50/50, ..., 80/20). To make the panel widths equal, double-click the Split bar.