This is an interesting discussion. Nobody has mentioned the reason I was expecting to find when I saw the thread title: shelf time. With the barrage of short PDFs coming out, the amount of time thumbnails appear on the category pages decreases. That means the 80 page/4 month project gets just as much (or little) exposure as the 5 page/1 week GM aid. With thumbnail "shelf" time being so important for sales on RPG Now, I don't find it surprising that more publishers are following suit.
Please understand I'm not complaining or trying to be inflammatory. It's just a part of business.