If you look carefully at all the maps, you will see many areas present, but shaded out. Even the area opened up for Crystal Tower existed on the map previously, but was shaded.

If CT is a precedent, many zones could be up to 4 times larger than they are currently. I mean, look at Mor Dona's map fully zoomed out, It takes up such a small space of the overall map that it just begs for expansions.

They can even modify zone change lines because no one really knows exactly how far your character traveled between the two zone lines. Looking at the World Map Black Shroud, theres an immense expanse of road between any given area. We may even see the path from say North Shroud to Central get an area between it that we zone into before walking through this new zone to get to the Central Shroud line, Coupled with each area having the potential to double in size, in a few expansions, we may see a truly massive world.