I realize about real world sizes, but I'm strictly referring to the ingame maps. Is the island of Vvardenfell larger than Cyrodiil's overworld?

Cyrodiil is significantly larger than Vvardenfell. That said, Vvardenfell's landscape is a lot harder to traverse, and your character generally moves slower; it can feel a lot bigger.

This

I think that this is all that there is to it. Cyrodiil just had a lot more blander landscape, so it seemed smaller.