To me it's not that much of a problem in size or complexity of the books - it is in the unnecessary complexity and authors evident lack of interest in writing/finishing it.
He has been doing this series for more than two decades and has published "only" 5 books - granted, those are some long books - but, in comparison, there are many other fantasy writers who have managed to write and conclude long(er) series in a lot shorter time period - I know that this comparison doesn't really have any importance, but I wanted to just point out the quite obvious lack of enthusiasm Martin has in all of this.
He has racked up millions by now, so why should a man of his age bother much with work? Not that I fully blame him. He told the show's producers how he plans to end it, so the problem of a series without conclusion isn't weighing on him anymore. Fans will get their ending, one way or another, all is well.
I just can't imagine how those die-hard fans who have been reading those books for the last 20+ years must feel by now - probably quite pissed off over being screwed over, since Martin basically shifted all of his interest from the books to the show (which I, for the record, haven't seen much, so I can't make a proper comment, but from all the things I've learned about it, it definitely isn't my cup of tea).