"As has already been said, length is going to vary a lot by discipline. My guess, @andrew99 is that you were looking at dissertations from a number of different disciplines, even though the same methods were used."
I was really trying to be very specific, because it would be completely unfair to judge every field under the same rubric. That's why I qualified it by referring to narrative works based on qualitative research. I do think, despite apparently universal disagreement above, that there should be a certain minimum level for a qualitative dissertation (outside some esoteric philosophy of logic kind of thing), and I don't think 70 pages should cut it.
"Last but not least, I just want to reiterate the point that length is not an indicator of quality. I think of it as similar to academic works. I've read some amazing original academic books that are only about 150 pages and some boring ones with minimal contributions that are 300 pages. Dissertations are similar. Perhaps if you really want to look for a correlation between length and quality, you'll do a study using a specific methodology to evaluate each dissertation."
I think once you get over a certain page length then yes, quality bears little resemblance to length. My argument is that as a gatekeeping device, even as a career qualification device, the dissertation is supposed to take a fair amount of effort. And if you're doing, say, a paper on the history of World War 2 in western Africa (to completely make up a random topic) and turn in something the length of a long-form magazine article, then you have not made the effort that should be required.
Some of the arguments above are "well, that's the Committee's job" which really doesn't conflict with anything I've said, unless people are arguing that Committee members are somehow above criticism.
I mean, my advisor is very much in the "the best dissertation is a done dissertation," and wants his students out, but even he would not let me turn in a 70-page dissertation unless it was quantitatively dense.
Anyway maybe I'm biased because I am a lot older than most other PhD students, but I've found over the years (and it took a long time to do it!) that doing things well rather than just doing the minimum to get by really does have impacts beyond whatever individual thing you're working on.