As someone who played through SA & V completely I can guarantee you V is BY FAR bigger than SA and surely the size of RDR, IV and SA together... RDR was not that big considering you mostly just walked or you were riding a horse...
I'm sorry but I disagree. Playing RDR, the map feels massive. I won't deny that GTA V's map is bigger, but it certainly does NOT feel like that and other R* maps combined.
Yes, the map feels massive. That's because you're on a horse, not a sports car.
No, it looks massive from the distance when you're on foot. Fail.
Cool. And half the time, you're looking at places that are out of bounds from the map. That's what makes it look bigger than it really is
The Mountains make GTA V feel bigger. Truth be told get rid of all the mountains besides Mount Chill and the map would be small.