Someone asked on Reddit how much space would the 32GB Microsoft Surface has available, not counting the space needed for the OS. Here is the answer from a Microsoft employee:
I am Ricardo Lopez, Test Manager for Surface RT. After the OS, Office RT and a bunch of apps, you will still have more that 20GB. As you say, you can always add an SD card and while you cannot add apps there, you can get music, movies and photos there.
So, the answer is that since Windows RT needs approximately 12GB of space, if there were a 16GB model it would pretty much be useless without an SD card. I know Microsoft wants to give users as close to a desktop experience on a tablet as possible, but 12GB?
iOS takes at most a Gigabyte (it’s less than that, but close enough).
And your point is…
Windows RT has much more inside of it than iOS does, and for better or worse that requires more space. Keep in mind that the 12GB also accounts for [probably] the Music/Photos/Email/People apps, plus the Office for RT.
If Microsoft gets around this by putting in more storage, and in the end that means that the user actually gets more end user space (32-12=20GB, still more than a 16GB iPad), plus the option of MicroSD expansion, why does this matter?