I do think they're supposed to be adding in optional airship and boat ride sequences eventually. But looking at the zones I think I can name two reasons they feel much smaller.
1.) They took one large zone and broke it up into smaller pieces. Look at a number of the zones, there's a Central Shroud, East Shroud, North Shroud, South Shroud, and so on. While the whole "area" is actually larger due to it being broken down into smaller zones it feels a lot smaller. Comparing the Black Shroud to say the Sarutabaruta in FFXI for example, Sarutabaruta's zones were larger but on the whole it was also smaller. The Black Shroud zones combined are actually larger. Same can be said for say Gustaberg vs Thanalan. I believe on the whole it's La Noscea that is by far the largest of the zone clusters compared to Thanalan and the Black Shroud. The whole world only feels smaller in comparison to 1.0 because they're broken into smaller, multiple zones.
2.) Teleporting is much more used than before. Previously to get around in 1.0 you'd spend much more time traveling via Chocobo or running as to avoid burning up all your anima too quickly. People who overused the teleportation system were then either stuck wandering around by foot all the time or saving their anima for necessary trips only. Now that teleporting can be done with ease but requires gil you're only limited by your source of income available. This in turn makes the world feel smaller since you can just teleport to any given region you want once you've been there.
There are two exceptions to the first reason however and that is Coerthas and Mor Dhona. While I understand Coerthas may be frozen off and opened later on I don't understand why only 1/3 of Mor Dhona is currently accessible. Is the other 2/3 of it completely inhabited by the Garleans? Was it utterly and completely devastated by the Calamity that it's too inhabitable for travel? The odd part is they don't call it Northern Mor Dhona but simply Mor Dhona nearly implying the small chunk of the zone we've got is all there's to it.