In Windows 8 you designed for three different views: 1) full, 2) filled and 3) snapped. In Windows 8.1, with the possibilty to have more windows on the screen at the same time, this is not the case anymore. Now we only count pixels. Your app could be 500px and larger. Snapped view, the view that are 320px is not longer mandatory.
So, it’s even more important now to have a user interface that can scale good between different sizes.
Here is a great presentation from Build 2013 that goes into more details. Beautiful Apps at Any Size on Any Screen.
Here is some updated documentation: Guidelines for window sizes and scaling to screens(Windows Store apps)
Why 500px as minimum?
The screen size for Windows 8 has to be at least 1024px. With 500px you can have two windows side by side.