I'm not a science-y guy, but from discussions with a couple of people in the propane industry, including an engineer who works for UL, it has to do with the boil off rate in relationship to the tank size and ambient temperature.
I've had problems running the burners WFO. The tank develops a frost line. According to those conversations, the tank is freezing because the
surface area of the tank can not absorb enough heat to compensate for the demand from the burner. This causes the pressure inside the tank to drop and reduces the output.
I tried to find a link that illustrates this, but my Google-fu must be broken.
Here's the best I can do. See the third table:
http://www.propane-generators.com/propane_usage.php
Edit to say the table isn't ideal because it assumes 25% full @ low pressure. I wish I could find a table for maximum BTU output/hr of a 20# cylinder but I suck.
Edit 2:
This gives an explanation similar to what I was told by Mr. UL propane engineer.
https://answers.yahoo.com/question/index?qid=20090124214947AAxUOqm