My hunch is the smaller the moka pot, the higher the pressure buildup, resulting in higher extraction. Is this true and are there any other variables that may negate this theory?
I would assume that size doesn't matter - if the amount of water is proportionally the same then the steam pressure should also be the same, regardless of the size.
But it seems to be much more complicated than that, and part of your answer probably lies in the research paper entitled "Experimental investigation of steam pressure coffee extraction in a stove-top coffee maker" published in volume 29 of Applied Thermal Engineering, which states:
Despite its well-established usage, the moka has never been the subject of detailed analysis, which led to a series of unclear descriptions or misinterpretations concerning its functioning...
That is if you can understand the thermodynamics, though the study itself seems to be a bit inconclusive. One of the closing statements is interesting:
The quantity of dry air can influence both temperature and flow rate, thus affecting final extract quality, and it is meant to be the subject of further studies
In the end, though, even the researchers decided that the moka is a bit of an enigma ("...a popular, yet mysterious, device, which so much diverges from other coffee-brewing methods"), so I don't know that you'll find any better answers outside of your own stove-top trials...