If we're talking pure size, I think you're correct, especially if you include the Shivering Isles.
Problem with Oblivion's gameworld is that the only points of interest tended to be copy-pasted caves.
There's also the fact that one thing I liked about exploring in Fallout 3 over Oblivion was that it brought back an element of the exploration that I loved in Morrowind which Oblivion lacked, that there was actually hand-placed loot, whether actually unique items or just nice items to find. In Morrowind, you could explore dungeons and potentially find some very worthwhile finds, sure, not all dungeons had them, of course, but the chance of finding such a thing alone made me really want to see what was in a dungeon when I found it, it didn't even have to be unique artifacts. In Morrowind, just finding a piece of rare, high level armor, like Daedric or glass or something, in a dungeon was rewarding, since you knew you couldn't find it in just any dungeon. In Oblivion, once you reach a high enough level, you can potentially find such things in any dungeon that has enemies or containers set to use the right loot lists, and with some very rare exceptions, all the worthwhile items you could find in Oblivion tended to be leveled, or quest rewards. Sure, finding good loot was still nice because it was useful, or could be sold for a good price if you didn't want to use it, but I couldn't help but find it less satisfying than when I just found some one of a kind item in some random dungeon. And Fallout 3 brought back an element of that, which I found quite welcome. Once again, if you looked in the right places, and sometimes if you had the right skills, you could find some very worthwhile things, one of the most satisfying experiences I could get when exploring in Fallout 3 was when I explored some old prewar building, cave, or other such place, and then found a unique weapon where I least expected it. I also liked that many of the dungeons had some actual backstory to them, this was of course most noticable in vaults, where the experiments that went on in them would often be revealed through various sources. But even normal buildings often had some little pieces of flavor text on terminals, in notes, or in other places that helped to reveal what had happened in those places in the past. The information didn't always go into great detail, sometimes it wasn't even really relevant to anything important, but still, it helped to make the dungeons feel more like they were places that actually once served some sort of function, rather than just being generic places created to provide players with places to kill enemies and find loot with no real justification for their existence beyond that purpose.
Of course, New Vegas still has an element of that, but I do feel that I can agree with the subject of this topic. In many ways, it improved on Fallout 3, I felt. In many ways, I felt Obsidian did a good job of preserving what Fallout 3 did right, while improving upon things where there was room for improvement. But in terms of the random, unimportant locations in the wasteland, I felt like many of them weren't as satisfactory as Fallout 3. This is not to say that I don't enjoy exploring in New Vegas, of course, certainly, I've found locations that I felt were worth visiting, it's just that I felt that there were more locations that felt ultimately pointless, or seemed like they only existed to provide fast travel markers, so yes, I'd say that in that aspect, the game made a bit of a step back from Fallout 3, still, there's a lot of things that it did better too. I felt that the quests and writing were done better, there are many more quests, and while this of course means that some of them are pretty short and straight forward, there are also quite a few quests that I found quite worth doing. Also, while the changes to gameplay tend to be small refinements rather than major renovations, many of them are ones I approve of. The reputation system seems like a better way to determine how NPCs react to you than karma. For one thing, since your reputation is calculated for each faction based on how you've acted towards them, you can potentially be seen as a complete monster in the eyes of Caesar's Legion, whereas to the NCR, you're the anthropomorphic personification of all that is good in the world, unlike in Fallout 3 where everyone was evidently omniscient and judged you based on your karma, even if they could not have possibly known of the deeds that earned you your current karma, and letting the player choose which faction to side with rather than having one group be the good guys and one group the bad guys and force you to join the former every time you play is a pretty effective way to add replay value to the game, as well as avoiding the situation where you're forced to side with a specific group even if you don't like them. I also like the idea of the crafting system and weapon mods, even if I felt that they could have been developed better than they were (I wouldn't have minded more crafting options, for one thing. And I'd have preferred more options to weapon mods as well.) but I'm not going to list every single change from Fallout 3 to New Vegas that I approve of, so I'll just stop there.