I was hoping for a succinct answer from someone who understood the physics. I did some google searches but didn't turn up anything helpful. The Wikipedia entry, for instance, does not explain why a larger cabinet gets louder. Your links explain the various types of enclosures which is interesting, but they do not answer my question.
I'll put the question in another way. Take for example the Outlaw subs, the LFM-1 Plus and EX. Same driver, same amplifier. The only thing different are the port sizes and cabinet sizes. I understand the ports, to an extent, and their resonance, and I get that larger ports will produce more output. However, in my understanding the port's output is limited to the lower frequencies of the sub's tuning frequency, around the resonant frequency of the ports. However Outlaw claims that the EX has 4 dB more output across the board, not just at the tuning point. From what I gather, that is due to the larger cabinet. I don't understand why volume of the cabinet makes a difference here. If I had to take a wild guess, I would say this is because the larger cabinet has less air air pressure and therefore less air resistance and so the driver doesn't need as much current to oscillate. I haven't found anything that confirms that though, or maybe I did but didn't see the answer because I didn't understand that jargon and didn't recognize the answer.
Also, I don't just ask questions without attempting to find an answer on my own first, and I don't think any questions I asked had a readily apparent answer to a non-expert, although I will admit the post that started this thread should have been more specific.