The Wii U was visible in a mock living room setup, and is surprisingly large.
Nintendo focused on the controller very heavily in its presentation, and didn't really talk about the console itself at all. Perhaps the prototype's size has something to do with it.
Part of the Wii's appeal was that it was unintrusive--part of its identity is that it is small and portable, and surely Nintendo wants to keep that image. While the Wii U's on display are obviously early designs, apparently they take up a good deal more real estate. From the looks of it, while its face is slim and almost identical to the Wii's, I'd guess its depth is comparable to an original Xbox 360. Check out my picture and see for yourself!