Thats probably cause it calculates folder size recursively and the Devs dont think its worth it to index each individual folders size.

File Explorer can help you organize your existing media files into libraries, so you'll be able to enjoy them from the Start screen. But it does show folder size in the properties. There’s also a smaller window that sorts size used by file type and switches on/off desired extensions in the graphic view. The apps on your Start screen are optimized for media, making it easier than ever to play music, watch videos, and view photos. You can sort by size, name, number of files, number of subfolders, date modified, When you click on a folder in the list, it is highlighted on the graphic, and vise versa. For example, any songs or albums in the Music library will appear in the Music app. Libraries are especially important in Windows 8 because many of the apps on the Start screen-including Music, Photos, and Videos-use your libraries to find and display content. To learn how to display them, read How to Bring Back Libraries in Windows 8.1. If you're using Windows 8.1, your libraries may be hidden by default. For example, you might include a folder of important spreadsheets in the Documents library while still keeping the folder on the Desktop for quick access. You can include any of your folders within a library without changing where they're stored on your computer. Libraries do not actually store your files and folders-they simply help keep them organized.

There are four basic libraries in Windows 8: Documents, Pictures, Music, and Videos. Libraries are collections of content that can be easily accessed through File Explorer.

Libraries make it easier to find your files whenever you need them. The Search Tools tab Working with libraries

Sort by folder size