In my case, Cyrodiil felt gigantic when I first started it up in 2008. But I was used to a multitude of games that don't allow much exploration. Games on PS and PS2, that is, with tons of invisible barriers. It took quite awhile to find one of Cyrodiil's invisible barriers. In comparison, a game like Crusaders of Might and Magic has invisible barriers as soon as we try to step off a particular path.
Nowadays, I realize both Cyrodiil and Skyrim are relatively small, but this is comparing to my real-life hometown. This all depends, though. I rarely fast-travel, instead preferring to walk from place to place. My characters usually only run if they're being chased by an enemy. I'm sure if I FT'd and ran everywhere the gameworld would feel much smaller than it is.

And I finally wanna say that there are areas in both Cyrodiil and Skyrim (unmarked roads) that can take hours to travel through. As my characters walk through these areas, the world
does start to feel larger than it is.