I recently did a full playthrough of GTA IV including the DLC and while the map is objectively smaller then V's because it's spread out into four different islands it feels much bigger to me then GTA V's map. The density of the city also makes it feel much fuller then V which has a mostly empty north.
It reminds of this:

STALKER: Shadow of Chernobyl was composed of a dozen surface maps with half a dozen underground maps, all of which required a loading screen to go in between. They weren't especially large, some of them weren't even a perfect rectangle of explorable areas, with the largest map being the ChNPP which got divided into two separate maps, and going from one map to another wasn't especially a long journey.
But it felt like one, because the world map had a lot of padding to make you feel like you're going through a massive journey, and to go from one end to another you had to go on foot and fight off any bandits, mutants and other hostiles, with the entire world design making you feel like you're desolate, constantly in danger, even though the gameplay itself is relatively easy and nowhere near as bad as the game makes you feel it is.
This is masterful game design, making a small play area feel massive and actually make it engaging. R* managed to accomplish that in IV since they were making NYC, a tightly packed city that has a lot going on in it, and they've populated it with plenty of things to do to keep the player engaged.
With V, they just made a massive map that was barren of any content. They decided to make the biggest map in an open world game ever, but failed to make it engaging. There is so little to do in V besides doing another circle around the state. Most of the map is taken up by Mount Chiliad, that again, has fuck-all to do on, the desert, being a desert, is fucking boring, and even Los Santos is boring.
Hell, an even better example. GTA: San Andreas.

It's not a big map by today's standards, it's amazing that they managed to pack it into a PS2 game back in 2004, but the way they've designed the map, and the game itself, it felt like a massive map, bigger than it actually was. The fog did a great job to make the player feel like the map is bigger than it actually is, so going from LS to the counties, then to SF, then to the desert and LV, it all felt like a grand discovery of a massive world.
And nothing shows better how important the fog was in San Andreas' game design to make the game feel massive and engaging than R* lazy cash grab outsourced to the same incompetent studio they've been using to port the 3D trilogy to consoles, where they completely fucked it up, removed the fog and made the render distance infinite.

Suddenly the world feels small, because you can see Mount Chiliad from fucking Grove Street. Get onto this highest building in LS which is accessible from the very start of the game and now the entire game is not only spoiled, but also exposed as being tinier than it was meant to feel like. And most importantly, it just looks wrong. Because it was never designed to be seen like that. It's a square map that has a specific design to fit the gameplay and the story itself, that the player is supposed to discover slowly. And the player will never see that it's a rectangle, and it won't feel like one. Plus, this smaller scope of the map's size allowed R* to more effectively populate it with activities and memorable mission locations so that it would be engaging rather than boring and tedious like the map in V.
You could write an entire essay on this single aspect of game design. The goal isn't to make a massive world, but to make a small world feel massive. Because a massive game world won't be memorable or enjoyable, but a small game world can be made very memorable and very enjoyable, without making it feel small. And you can show it with GTA games alone, where R* nailed it in 2004 then completely fucked it up in 2013.