Yeah, I always found that fan map comparison highly doubtful...
The game world still seems to be very big, probably still bigger overall than GTA5's island but probably not bigger than The Witcher 3, especially after it's expansions.
The in-game map in your screenshot just depicts Lucis with it's regions of Insomnia, Leide, Duscae and Cleigne (seems you didn't visit this one in the Gamescom demo?) and that probably makes up about a third of the complete game world. But that is only my guess here. Beyond the ocean it goes of course on with with Accordo,Tenebrae, Solheim, Niflheim etc. And some places we don't know of yet. The developers said that we haven't seen more than half of the regions and cities yet that are in FFXV. So, there might be some cool and interesting stuff for us ahead. The more fantasy-inspired regions and cities are also said to be waiting near the end of the game.
Wow ok, that's actually pretty exciting to me. As long as they iron out as many kinks as possible.
Not to mention that literal size and perceived size are very different things. Where in GTA you can pretty much go anywhere anytime, it feels massive. But since most of your time with 15 was on car (i assume, please correct me if i'm wrong) on simple roads (not clogged interconnected roads like some areas of GTA) no less, it could very well feel like it was a smaller area.
It could very well be that the Density of witcher and GTA's worlds make them feel bigger and the design of FFXV's world makes it feel smaller. World designs are very different so the feeling of size is different.
This definitely could've had an affect. I was on foot for a few different sections but for the most part is was driving.