I don't think voice acting caused space issues so much as all the casting resulted in a lot more time constraints for Bethesda. Look at it this way:
Morrowind (Developed ~1998-2002)'s world was done entirely by hand. Bethesda had to learn the Gamebryo engine's tech and create the Construction Set.
Oblivion (Developed ~2003-2006) was sorely lacking in variety despite having a similar development period. They already had the Construction Set working, and most of the new tech (Havok, SpeedTree) was middleware. They even generated Cyrodiil via heightmap instead of doing it manually like they did with Vvardenfell. I like the physics and radiant AI, but I don't feel as if they're an equal tradeoff with everything that was lost/downgraded. Especially considering that Bethesda should have been far more comfortable the second time around than when they made Morrowind. :shrug:
It could be because of the casting, I'm not sure, they always have the same people voice in all the games, I'm skeptic about it, because there's really no way to tell, we don't know how much had to be redone. I think the overall lack of variety (even though there actually is some), had to do with them having difficulty translating their ideas into actual practicality, specifically refering to the "radiant" A.I. On top of that, they had short deadline, I hate deadlines, when ever I see something which doesn't make sense to not have, I usually blame the deadline. For example, in the emperial city, there's a feud between two families, but it's not fleshed out at all, you don't actually see it or experience it, it just kind of hanging there, like a sentence left unfini
I think Beth should do like Blizzard, and simply say, "it's done when it's done".