There has been no attempt to keep each game in scale with its predecessors.
Daggerfall, a fairly small province, ended up at "life size" (several hundred miles of mostly emptiness).
Morrowind, a sizable "U-shaped" province with a large island (Vvardenfell, where the game took place) in the middle of the bay, was heavily reduced, so the supposedly 100-200 mile island was more like 3 miles. The entire map was "playable" (in all 3 dimensions). We never visited the mainland, other than the quarantined center of one city in the Tribunal expansion.
Cyrodiil (in the game Oblivion), the largest province on the continent of Tamriel, was supposed to be substantially larger than the entire province of Morrowind, but ended up only marginally larger than the island of Vvardenfell. According to lore, the Imperial City Island alone should have been nearly half the size of Vvardenfell, which means that the game could (and perhaps should) have been restricted to that island, rather than cramming the entire huge province into a nutshell. Thanks to unclimbable mountains, border areas you couldn't access, and other restrictions due to the removal of Levitation, the actual "playable" area may only have been comparable to Morrowind's, and there was practically no "vertical" aspect to the game.
Skyrim is another sizable province, and despite being slightly smaller than Cyrodiil in lore, was supposedly a bit bigger in-game.
In essence, Valenwood, Elsweyr, or any other province, can be, and has always been, whatever size in-game the developers found "appropriate" for the game mechanics. That means there is no reason to combine two small provinces, when each of them has more than enough variety, content, and culture to require the full attention of the developers in order to pull it off well. I'd MUCh rather have two well-made provinces, each in their own game, than to have one game with two cultures each "half done".