User blog comment:Kacj321/Entire Tamriel Landmass built into Skyrim/@comment-121.222.63.120-20120401015833

I do believe this is because, while in their early days of Arena, Redguard etc. they already had the heighmaps etc. plotted out in their game engines - the landmass is already there (Even in Morrowind, Oblivion etc.) but it is just not detailed. ;)