I've seen the trend in my own work. The editor asked that the piece you're now reading be 600 to 700. We still read epic novels, as well as histories and biographies that can double as doorstops. Think of all the reading that comes in chunks of only a few hundred words. Seven hundred or so is the standard for columns and many articles in the daily papers.
Most web pages are shorter yet.
Ditto many reviews, and the front-section pieces in most magazines, and the little boxes into which larger spreads are broken. We need a name for this chunk of data. It's more than a bit or a byte, but it's not a lot. You do a candy bar, a couple of Taco Bell tacos, an ice cream and wow, you feel bloated. First, you're not getting much depth.