So the thread devolves into a string of token comparisons to Pete Molyneux (please, someone give me a source of when he ever said that he was worried that his games would be too big) and token "jokes" that MGS is a movie and not a game become hurr hurr a series that has always focused on its story before it was even something to complain about is terrible hurr hurr etc, etc.
I can see where Kojima is coming from. His series has always been hyper-linear, even MGS3 which was the most "open-world" the series ever was until now. Throwing a huge map into a stealth-based game is going to increase the amount of time someone plays it (imagine if you had to sneak all over Skyrim all the time).
There's a trope for what Kojima is worried about, and it's the Quicksand Box [http://tvtropes.org/pmwiki/pmwiki.php/Main/QuicksandBox].
Being more into linear games, almost every sandbox game is a Quicksand Box to me, and considering that fans of MGS expect a linear game, I can see a sizable chunk not wanting to bother with an open-world stealth game because the slow pace coupled with a lot of alleged content runs the risk of making everything a crawl, and that's not even taking the story into account if you don't skip it, which a lot of fans won't.