How to reduce space between mailbox in Folder Pane, Outlook 2013
It is because Outlook 2013 defaults to the iPad mode so there will be more space between folders for the fingers. If you right-click on the down arrow by the Quick Access Toolbar in the upper left corner, then select Touch Mouse mode, it will put the little icon on the panel. Then select the icon and select Mouse Mode. That will close up the panels considerably.