Also what exactly do you mean by 'a game this size'? Are you referring to the amount of space the game will occupy, or the size of the game world? Because we don't know the game installation size, as far as I'm aware, and having a large game world is no excuse for poor graphics (though the screenshots are by and large impressive).
What he is referring to is not only the size of the game world, but the number of objects with which the player can interact.
Crysis (or any FPS for that matter) is mostly a "linear dungeon" game. You go from point A to point B on a specific path, whereas in games like Skyrim you'll be able to run around the entire map on any direction at any moment.
Also, most games do not have the number of interactive items a TES game like Oblivion has or Skyrim will have. Most games have specific items the player can touch, break, throw, bump into, etc. For example, if you played Dragon Age Origins, your character wasn't able to pick up anything off a vendor's table, as non of the items on the table had collision meshes, and a lot of these "itmes" were merely painted on the table's texture; whereas in Oblivion, you ran by a table that had plates, cups, etc and they all went flying into the air, and you could pick them up, kick them; or books on a book shelf, or fruits on a vendor's cart...
All of this rendering of hundreds of objects with collisions puts heavy strains on system resources, just as rendering highly detailed, large size textures/bump maps, or meshes with high polygon counts.