Wagner, Malazan book of the Fallen is bigger in scope and has just a wide range of characters that die off.
I think the big difference is that many authors that do big stories like Game of Thrones, will often write a very detailed plan before they publish. So that even if not every book is written out, they have a clear written setup of how each book will pan out; they just need flesh adding to the bones as it were.
With Game of Thrones its evolved over time somewhat and I think Martins early plans were perhaps not as tight as might be. Also don't forget he wrote out a huge time jump he was going to include so a huge bunch of stuff is already thrown out from his earlier plans.
You can see publish-as-you-go affects all authors. Info gets forgotten, little and big things shift around. Even writing style can change. Compare something like The Colour of Magic to Pyranids or Going Postal.
edit of course it should be noted that even a very detailed plan can come up with problems. Any author can miss things that add up from earlier books to cause problems in latter books; and lots of little things can add up to big problems.
The real issue is that he's already set in stone the early books. So he can't go back and chop and change. Of course this has a bonus, already publishing material brings in income, gets the name out there and also sets things in stone otherwise some authors can get stuck in a rut forever changing.
But the big downside is he can't go back and edit any thing. He can get away with some subtle changes here and there; or present some characters as telling lies earlier or getting information wrong for glossing over details (Robin Hobb does this latter one when at the end of her first series her lead character goes on a jaunt around part of the world he lives in and describes, very briefly, a few areas that are later major areas - one being a river that he casually navigates that is later proven to be highly acidic to the point it destroys wood - a fact later elaborated on by that character with a tiny bit more depth. She gets away with it because in all relation to that character the interaction and description were very casual and short