This post deals with the question: “Why can’t I see the size of folders in Windows Explorer?” The simple answer is that the behavior is by design. You can view the size of folders by hovering over the folders in Windows Explorer (as shown below):
The primary reason why the Windows UI does not provide this functionality is performance. Every time you navigate through a folder structure in Windows Explorer, the OS would have to perform a recursive scan of the subfolder structure within each folder to get the file and folder sizes and add them up. This would create significant processor overhead – especially if you think about scenarios (home folders for example), where the nested directory structure for each user may be several levels deep and consist of thousands of files and folders.
More info: Performance Team