I don't know.
There are games that do exploration well, but the rest of the time it seems to exists only to excuse a lack of content. Oblivion/with-guns smacked quite strongly of this. The quantity of content in morrowind was excused by the limitations of the format at the time. But subsequent games were expected to have more of everything and exponentially more content per square yard. Instead we got bigger worlds with less content and some dodgey voice acting. Oblivion-with-guns, for example, becoming more the search for more dialogue from the only decent talent in the cast than anything else. While all the side-content is just that. Random crap with no relevance to the story or purpose in establishing the world as real.
Another example of over-priced fluff standing directly in the way of making a bigger game with more to explore and do is mass effect and it's sequel. Instead of paying some guy to read off a couple thousand pages of flavor text, how about some more variety in missions, planets, and environments with more plot missions and missions that directly establish whats written/spoken in the codex?
We have the tools to make living, breathing worlds. In these worlds, hundreds of pages of text can be compressed into a single observable event. The bachelor party in me2, for example, gives more information and context on the asari than EVERYTHING written/spoken in the codex.