Many people have written me to ask why I put in my notations in an earlier post about word count in new fiction needing to be in the 70K - 100k range, with certain genre exceptions. Yes, it is true that a lot of fiction violates these rules. Yes, some stories take longer to tell. Yes, famous authors have written much shorter stories. The real reason behind it is not about us but about the current demands of the publishing company. I'll explain it as best I can.
There's a certain overhead for publishing a book. There's costs for the binding, the cover, any editorial work, the cover art (usually done by an in-house artist or contract employee), and publicity. These range from book to book (especially in the publicity area) but stay the same regardless of the size of the book, but it is very expensive to publish a book. The overhead is huge.
If you talk to publishers today about why the price of books always seems to be skyrocketing, they will go on and on about the rising cost of paper and ink, the two things that are affected by word count. If the book is too short, they can't charge as much for it (because it's small) and may not recoup their basic overhead costs. Oh yeah, and they have to give a portion to that writer too, or something. They have to consider the public, which often feels cheated if a book is too short, the same way people feel cheated if a movie is an hour long.
On the other hand, if the book is too long, paper and ink starts becoming a major concern because the cost of the book to print will go up and exceed the price they can reasonably sell it for. This doesn't count for Harry Potter, as everyone will buy the 8oo-page volumes the day they come out anyway, but if you look at the first three books (before JK Rowlings became a major success in the States), you'll notice that they are much shorter than 4, 5, and 6.
So, the publishers have a comfortable range that they think a book belongs within, which varies based on genre, and if you want to get published, you should stick to their rules. Once you're a famous author, you can do whatever you want, but let's take it one step at a time.