The experience of reading a long novel and reaching the last hundred pages and realizing the author has more story than they can resolve in the space remaining is one of the more frustrating reading experiences. You feel the narrative contracting when it should be expanding. Characters are rushed toward resolutions that haven't been adequately prepared.

This is different from a novel that takes fewer pages than expected to say what it needs to say — that's economy. I'm talking about a novel where the author has made commitments the length doesn't allow them to keep.

The test I now apply when choosing long novels: do the reviews mention that the ending is unsatisfying? If the consensus is that the first two-thirds are excellent and the last third disappoints, I've learned to trust that consensus. Some novels that I loved despite their endings I would have loved more if I'd known to adjust my expectations.