World scaling...
The lake in the lore is much, much, much, much, much bigger. Vvardenfell is smaller than Skyrim lorewise, however because of world scaling the world of Morrowind is significantly bigger than the world of Skyrim and that even though Morrowind was scaled down quite a bit compared to Daggerfall.
This has always been my take with the ES games. After all, if we take the provinces' size at face value, they would be about 10 miles x 10 miles. All of Tamriel would fit inside of Rhode Island. It just is not possible to make a game world that is the size of France or Great Britain at a 1:1 scale. Since the lake outside of Riften is big even by game standards, I imagine its real world equivalent would be something like Lake Baikul or Lake Superior.