Established writers who will get sales because they are who they are get carte blanch in terms of length. For some, this is liberating; they need not stifle their brilliant riffs. For others (cough-Stephen King-cough), an editor demanding a reduction in word count would have produced a far better novel. But if it sells without it, the publisher is not in any position to complain.
To be fair to King, this seems to be a problem with lots of bestselling writers who suffer from fearful editing -- IMHO Rowling's last three Harry Potter novels could have been easily cut by more than half and been better for it (and one of them, Half-Blood Prince, could probably have been cut altogether from the series and not have been missed).
And in King's defense -- even though I think he's produced more misses than hits overall, even when he was well-past the need to submit to editing he has produced the occasional slim novel (The Girl Who Loved Tom Gordon and Gerald's Game are two that come to mind) and his lengthiest novel (The Stand) is generally considered to be his masterpiece.
On the flip side, some authors do become lengthier as they go along because they become better writers and produce more complex novels -- compare Dennis Lehane's early Kenzie & Gennaro mysteries to his novels from Mystic River on (his newest is 720 pages and getting great reviews).
For those of us who are aspiring but unpublished novelists, I think it stands to reason that lengthier manuscripts are harder to sell, even if there are occasional exceptions.
Perhaps my 180,000-word opus is a work of sheer genius and utter brilliance and will break all the barriers, but I just don't trust my judgment enough to risk wasting that much time.
