Does anyone have figures on units sold vs. larger PDF runs?
Do they sell as well? (volume as opposed to profit)
The smallest/cheapest products I've done are my spell books and the 22 Talent Trees. I've done the math on prospective titles, and I know what sort of sales it takes to attain various rankings on the RPGnow site (rankings held by some very short ($1.00) products right now.
Speaking strictly in terms of economics, it is the best way for a publisher to turn his time into money. The bigger your book, the more time you have to spend writing it. Of course you can charge more for it too, but the justified increase in price does not make up for the extremely long increase in production time. For example, I'll use two of my own titles to illustrate my point.
It took my roughly 6 months to write the Primal Codex (120 pages, sells for $10). I've made a little over $2,000 off it as of right now. So that's $2,000 for 6 months of full time work (56 hrs a week or more, no days off), or roughly $11/day.
By contrast, I wrote 22 Talent Trees in less than 2 weeks (and could have done it sooner if I wasn't working on other things at the same time). It has also sold over 200 copies, but at a mere $2.88 each. So that's roughly $450 earnings for 2 weeks of work, or a whopping $32/day.
This happens because the smaller products are more expensive to the consumer when you consider the price/word count ratio. With the Primal Codex, you get over 80,000 words for $10 or roughly .00125 cents per word. By contrast, 22 Talent Trees is only about 8,000 words, for a price/word ratio of .0036 cents per word. The words in 22 Talent Trees are almost 3 times more expensive than those in the Codex!
Therefore, writing small titles that you can pump out in a week or two is a much better way to turn time into money than writing long books.
That being said, I have a few other points to make about this practice.
1. I'm not the only publisher that has figured this out.
2. Although I figured that out a long time ago, its not really my bag. I've done some smallish books, and will do more in the future when it suits me. However, I'm also going to continue doing the much larger books as well because I know that is where the value is (at least to me, as a consumer).
3. The fact that I've figured out how earnings work for small titles compared to big titles means that I haven't bought any small titles. I'm waiting for them to be collected into an Anthology or bundle and sold at a more reasonable price/word count ratio.
4. Dave Stebbins makes a good point about the usability of the smaller titles. Basically, he’s saying that the consumer gets just exactly what they are looking for (however small that is), with no other material at all. Apparently people are willing to pay more for that, or they don’t realize that they are paying more in the first place.