So Night contains all the issues of Batman and Robin, Nightwing, Batgirl, Batwing and so on that had their own brief stories—generally just an issue, maybe two—that tied-in to the "Night" story arc. I was pretty surprised to find it contained four issues of the Snyder/Capullo Batman title, and the first New 52 Batman Annual (co-written by Snyder), as those seem like they would be issues that should be collected in the second volume of Batman, not in this huge, companion/check-list collection.
I remember thinking that was pretty weird at the time, but ultimately just assuming DC must have chosen to collect those there because they thought the book wouldn't make sense otherwise without them (Similarly, DC has just released Batman Vol. 3: Death of the Family and Joker: Death of the Family and are starting to release all of the Batman books that had "Death of" tie-ins, most of which take "Death of the Family" as their sub-titles; all of them that I've seen so far include Batman #17, the final chapter of the story, because that provides the conclusion to the cliffhanger ending of many of the tie-ins.)
This month DC released Swamp Thing Vol. 3: Rotworld: The Green Kingdom (not sure why there was such a delay between the two, as they occur simultaneously, and tell the same story from different points of view). That contained seven issues of Swamp Thing and two issues of Animal Man. The way the story is structured, the two characters meet at the beginning, journey to Rotworld together, are separated from one another for a stretch of time, and then reunite at the climax. And, while I didn't count pages, a lot of material appears in both collections, because it is necessary to the story, although neither story is actually a complete one (remember what I said about Animal Man Vol. 3), without the missing parts.
It's a very strange way to collect these comics which, when they were originally published serially as comic books, they crossed-over with one another in a Superman triangle sort of way—you would read an issue of Animal Man, then one of Swamp Thing, then one of Animal Man, and so on. The entire point of such a publishing strategy, when it comes to the serial comics, is that it encourages readers of Animal Man to also pick up Swamp Thing, and vice versa (and, hopefully, readers of one will like the other enough to keep reading it when the crossover ends).
But the way the trades are published, they seem to be collected under the understanding that those who read Animal Man in trade probably won't also read Swamp Thing in trade, and vice versa (Which likely isn't true; aside from this storyline, the books share a lot of common ground, including starring characters who went from quirky DC superheroes to dark, mature horror titles under the Vertigo imprint, to recent New 52/DCU emigres; both are also written by Vertigo writers-turned-New 52 breakout stars).
So, if you are reading both titles in trade, and certainly that's what DC would want you to do, if you're not reading them both in serial, comic book form, then you're getting a significant overlap of material in the two trades, which has to be frustrating giving the cost of a graphic novel these days (These are $17 a pop).
What they ultimately decided on was collecting it as Justice League Vol. 3: Throne of Atlantis (collecting Justice League #13-17 and Aquaman #15-16) and as Aquaman Vol 3: Throne of Atlantis (collecting Justice League #15-17 and Aquaman #0 and #14-#16). The "Throne" storyline thus appears in both books, the colletctions having about 100 pages of identical material in them. Given that they are about 140-pages of story content apiece, that's a pretty significant story overlap, and given the price of these hardcover editions $24.99, that's gotta be maddening if you read both books in trade (And, again, these are both by Geoff Johns and both feature Aquaman; chances are, a lot of folks who read one in trade also read the other in trade).
So say you do buy and read both of these series in trade and you learn that these two volumes will mostly feature material in both, what do you do? Well, you could obviously just buy one, but there's still a problem, as their contents are mostly identical, but not completely so.
Now, if you were reading both books as they were serially published, none of this matters. But if you were trade-waiting both of them it sure does.
I'm not sure why DC does this (I suspect this is also what happens with some of the Green Lantern books and will happen once the three-book Justice Leagues crossover "Trinity War" starts showing up in collections). It may be to encourage purchase of the monthlies in the future, by punishing trade-waiters, but that seems rather unlikely. It may simply be that the folks in charge of the comics and the crossovers don't really worry about how they're collecting, and then a different set of folks has to try and make sense of some way to collect them while still having generally complete-ish stories in each collection.
I think a better way to do it would be to excuse crossovers from the volume numbering, and run them under the title of the storyline or the characters involved. In the case of "Rotworld" then, it would have probably made more sense to publish a book collecting all of the relevant issues of both books and called it Animal Man/Swamp Thing: Rotworld, or simply Rotworld. In the case of "Throne," a Throne of Atlantis collection might have made the most sense, or perhaps a Justice League: Throne of Altantis, with no volume number attached.
I don't know, though. It's a pretty weird problem and one I can imagine being extremely frustrating for trade-waiters, and one that doesn't seem to have a completely perfect solution.