I haven't played GTA V yet, but it having a world almost twice as big as Oblivion sounds insane to me.
HA!

You think that's insane? Wait 'till you see and explore GTA VI's map! Said terrain's spoken to cover about 125 sq kilometers, and about closely 3x bigger than GTA V's map. Don't know how it is compared to previous open world games, but, that'll hit the spot as the biggest open world map yet... unless another game says otherwise.
On the other hand, there were the well-known Batman: Arkham games and its follow-ups that had their maps bigger than the previous, but they've been behind in their sizes compared to GTA's games.
For Arkham City, it was made five times bigger than Arkham Asylum, which, is kinda like a confined world of its own, but technically holds the makings of one. Then, Arkham Knight, which arrived later, made it, well, five times bigger than City, as well, to better accommodate anyone being able to ride the Batmobile across it.
Small fact: There was a prototype Batmobile made to see how it would flow well driving into Arkham City's already existing map, but the city's size was claustrophobic for Batman to glide and using grapple didn't work well. Meaning, the city was too tight to drive the Batmobile in that map. So, the city was redone with wider streets and taller buildings to accommodate ejecting the vehicle.
Don't know how big Sunset Overdrive's map is, but some guessed it's about 8 square km, bit more than Arkham Knight's.
Later came Gotham Knights, said to be bigger than Arkham Knight's, but Suicide Squad, which got out later, too, also had its map bigger than that. Don't know if SS is bigger than GH, but I hope for a comparison to see which is bigger than the other.
And, last, uninvolved with the whole Arkham series, there is another next game in the LEGO game series from Traveller's Tales, Lego Batman: Legacy of Dark Knight. That game is primed to have its map much bigger than Arkham Knight, even quite moreso than SS's map.
Funny you should say that since moving around is how you increase some skills in Oblivion. Considering also how much inventory (weight) you can carry the player must get a real workout. 
Makes us wonder how other heroes, or even villains, if they have no vehicles to go around, have felt they've done the same after running through all those more and more bigger landscapes in each of their adventures.