That size in the Map is not THAT big, by bird flight it only take a few (a dozen, actually) seconds to cross any of the areas shown in the map (you can stimate that by counting the time it takes to Link reach the green beacon in one of the previous videos).
I mean, it is big(any of the provinces), but not enough to hold another 2 hours long themed areas like the Eldin Volcano area.
But... yes, I believe that in each of those areas we will have other sections (that might represent some 30% of the province) besides the pre-dungeon areas, some of settlements (areas with some npc's, shops and mini-games), some special more pacific areas around it, with even some theme variation in it.
A "water province" is not confirmed yet, it might be inside that big lake/sea we saw in the desert province (the water in the boat clip seemed clear enough to suggest that we'll be able to dive), or it might be some recurrent element that will appear in all 3 areas (flooded areas. lol, that would be boring), since we saw a absurd part of the forest area submerged in one of the videos.