(Feel free to think “big” as size, or as popularity, or in any other way you care to interpret.)
I think that if I read through a chunkster (which I love BTW) it has to be be "big" in every possible meaning of the word. It's big in size, big in meaning, big in popularity, big in importance, etc. but especially big in story. I don't care about ambitious novels written in stream of consciousness or just plain gibberish (I have read them and I will not come back to them any time soon). If a book is over 500 pages, it had better give me a darn good, engaging story.
There are two books which I consider to be 'big' and have read them fairly recently (not this year though, with all the review copies and challenges to complete, I had no time for re-reads):
1. The Stand by Stephen King - all time favorite and a post-apocalyptic classic. As a fan I can only recommend the Complete & Uncut edition counting 1200 pages in hardcover. I know it's huge but when it was first published, The Stand was cut by over 150,000 words and I want a whole story :D. One thing I should mention is that because it's the end-of-the-world masterpiece, everything else I read after that just doesn't measure up.
2. John Adams by David McCullough - one of the best biographies I have read. It is another chunkster with 722 pages in paperback edition but yet again, really worth the time and attention. I loved it because the author is a born storyteller, the story of our founding fathers and the birth of our country is beautiful and reminded me how many lives were put on the line so I could live in a free, independent and democtratic country of the United States.