From Business Insider:
At first blush, Microsoft's entry-level Surface tablet seems like a good value compared to the iPad and other tablets. That's because you get double the storage (32 GB versus 16 GB) for the same price as the iPad, US$499 (C$519).
But the reason Microsoft started the Surface at 32 GB instead of 16 GB is because the operating system,Windows RT, takes up approximately 12 GB of space.
Those numbers come from a Reddit AMA session with members of Microsoft's Surface team. When asked how much space Windows RT takes up, Microsoft's Surface test manager, Ricardo Lopez said there will be about 20 GB of free space after Windows RT, Office RT, and "a bunch of apps."
It's not a straight answer, but it's pretty clear Windows RT is a massive operating system. For reference, Apple's iPad operating system takes up less than 1 GB. If Microsoft had made the entry-level Surface a 16 GB device, you'd only have about 4 GB free to play around with.
Remind me again why I should care about anything Microsoft does?