How about Bethblog?
http://bethblog.com/index.php/2007/09/27/inside-the-vault-mark-lampert/
"Yes, it would seem that our designers know no bounds when it comes to writing for our games. Also, consider this: at one point in development, there was almost twice that much! A figure was calculated and we knew that we could never fit that much into the game, so they all worked hard to hone it all down and still have it turn out so wonderfully."
- Mark Lampert
Where, exactly, is it stated that DVD space was the reason? It could have simply been that the fee necessary for voice acting was too much for Bethesda. It also could have simply been time constraints limiting the time need to model all the characters or record all the dialogue for the writing. The fact of the matter is that Oblivion, having taken up only 4.6 gbs, did not fill up an entire DVD. A dual-layered DVD could, at the time, hold up to at least 8 gbs of space. Mark Lampert stated that the writers essentially got carried away with all the writing, not that they had recorded and implemented voiced dialogue but couldn't get it on a DVD so they had to scrap it. He never said anything about DVD limitations and if he did, he would be a liar. You can pretend DVDs are magically limited to only 4.6 gbs of space, but that doesn't make it so.