I would think they would make the original O/S hold more space than is necessary so if the O/S did become bigger, it did not effect the user in anyway. Someone who has already filled up their basic and has to delete things to upgrade would not be happy.
I also find it ironic that the Wii U is flash memory, but Nintendo recommends you not use flash memory. Instead they want you you to use an external hard drive for the Wii U because flash memory is less stable.