Of course the map will be filled with enough content. I doubt that there will be an excessive amount of dead space in GTA 5. Its just going to be natural. Take the wind turbines for example in GTA 5, that could be considered "dead space" but it not only serves as a good backdrop but it also is a realistic aspect to the the environment.
So if skyrim were much bigger Im pretty sure bethesda can match the size with content just like they always do. But I'm under the impression that you actually want every single part of the map to have something to do. And that has never been the case with any of Bethesda's games not to mention would be very unnatural.
I just don't see how much bigger Skyrim could be. The Elder Scrolls games already take 5 years to make. They are huge undertakings. If you add even more content it's going to take even longer for the games to come out.
You do realize there is over 300 hours of content in Skyrim, right? How much more content do you really want? Plus they are still expanding the game with DLC. Honestly, the amount of content in Elder Scrolls games is unmatched. While I would love more, more, more (I am a bit greedy), I am also realistic and realize you're probably not going to get a game with more content than Skyrim from the Elder Scrolls. Unless the next game has an even bigger budget than Skyrim, which may be the case, that all remains to be seen.
And how many of these Rockstar locations are hand-crafted? Is every interior hand-decorated, hand-crafted like in Skyrim? How many assets are being reused? Is GTA 5 going to have a lot of dead space? It's possible if the game is that huge. That all remains to be seen as well.
I'm just happy with the amount of content we get from Elder Scrolls. It's a pleasure playing these games and being entertained for a ridiculous amount of time.