This game journalist says that 60 hours into Dark Souls, that's only a fourth of the way through. He doesn't say anything about the map or world, which are smaller, but he says that it may be bigger in terms of content. Content can have more to do with the size of a game than the map dimensions, but from how it sounds you might have less freedom in an environment as brutal and soulless as Dark Souls'.
Still, could this be true? I haven't played Demon's Souls, and Dark Souls is open world unlike its predecessor. However, I really doubt that the world will feel as big and as replayable as Skyrim. They say they have put 20 hours into Dark Souls, but how much have they played Skyrim? Between the instanced content, guilds, and scale of Skyrim, it's hard to believe that a game which takes 5 hours and frequent deaths to undo one spell isn't just trying to span out the content, rather than create like an impressive and fully realized world. Could this journalist just be exaggerating for hits?
EDIT: Counting deaths and ridiculous respawns as content seems off - perhaps he should time how long it takes to get through the game if you don't die, and compare that number. It seems ridiculous for an OXM dude to directly compare a game he has played for 20 hours with a game he has maybe played for an hour at a gaming expo. I'll probably pick up Dark Souls, but honestly this article reeks of inception.