In Windows 7, by default, Explorer’s sidebar stays the same size unless you click within it. That is, if you use the right pane to navigate through folders, as most of us do, the left sidebar doesn’t change. With a simple tweak, though, you can have the left sidebar focus on your current folder in the tree as you click around the right pane. To enable this feature:
- Open up Explorer and go to Organize > Folder and Search Options.
- On the General tab, look under “Navigation Pane” and check “Automatically expand to current folder”.
Make Windows Explorer’s Sidebar Expand As You Navigate Through Folders | Lifehacker Australia