When you exit fullscreen the window can no longer be resized. It is stuck at either the default size or the size passed in with the command line parameters.

The way to fix this is to make the Display resizable after you set fullscreen.

Display.setFullscreen(fullscreen);
Display.setResizable(false);
Display.setResizable(true);

It needs to be set to false first because the implementation caches the last value and will not update the Window if it is already resizable in the cached state.