If you look at JC3 map and compare it to JC4, JC3 is tiny. There's also barely any diversity in the land with the only biomes being mountains and plains and small amounts of sandy dirt stuff near the mines.
The total area of the JC3 map (if the empty sea is included) is about twice as large as the other games, but the size of dry land is probably the smallest.
This might be because at one point during development they considered adding an explorable underwater environment, similar to GTA5, where the player could scubadive and see large sunken ships. The map was likely designed with this in mind to make use of the water. Or maybe they just felt like putting some more space between the islands that time.