I'm no expert, but I'm pretty familiar with this type of armchair analysis on TB. You need to look at the cab and the driver as a combined system. Looking at the driver, alone, gives you limited understanding of how the porting and cab volume come into play. Efficiency and sensitivity are related but not identical. In general, bigger boxes will produce bigger lows, given the same input. So the SM being the smallest, can still produce those lows, but you need to push more watts into it because the nature of the design is less efficient in the low end.
It helps to break the frequency bands apart. If you've ever looked at a mix frequency analysis, you'll see a "balanced" sounding mix has emphasis on the low end, and tapers off in the highs. This is because human perception requires lows to be louder in a measurable dB sense, for them to sound as loud as mids and highs. It's the way out ears and brains are tuned.
Couple that with the physics of how sound is produced and measured. You get a single numeric value of say 100db. That is representative of all the frequencies going on when a pitch is produced, and doesn't tell you the frequency that measurement was focused on. There's no free lunch, so if you want a cabinet to produce more low dbs with the same wattage, you've got to take away dbs from another frequency band. You may still end up with a 100db volume measurement, but it won't sound the same. Schroeder cabs are reportedly loud, as they pull emphasis from the lows and push that energy into the mids. The same wattage produces more perceived volume, but at the cost of lows. Acme cabs are the opposite, and known as inefficient, because their goal is to produce bigger lows. The cost for doing that is taking energy away from producing measurably loud sound in the mids, and directing that energy to the lows.
Now layer in cab design. The cab volume plays a role in the volume game. Bigger cabs are more beneficial for producing bigger lows, given the same input, with the same driver. The little SM has to use watts to produce extra lows that the SC and BB2 already have a tendency to produce because of the cab volume. The driver can take the extra watts, but you've got to feed them into the system. That's part of why the SC and SM are "equally loud" in a measurable sense, but don't necessarily sound that way. The SM sacrifices lows to create more total volume that competes with the SC, but it sounds less full doing it.
As for the tweeter, the 20khz rating is a measurement of what frequencies can be produced, not a measure of how good they sound, nor the dispersion characteristics. The SM tweeter is nice, but the BB2 compression driver is nicer. They both produce the same range, but the BB2 does it better.
I'm sure I've oversimplified something here, or crossed up some ideas, so correct me if I'm wrong. I'm sure someone will, since this is TB. I know Alex watches this thread, and is certain to put me in line. He's done it before, but I don't mind, as I appreciate the education
Edit
I almost for got to tie it back to these three boxes. There is a decent difference in physical volume for each cab, explaining why they each behave differently. Additionally, you've got two cabs with tweeters and crossovers. If you split the job of frequency reproduction into two sections, with one section routed to a woofer (optimized for producing lows) and one section going to a tweeter (optimized for producing highs) then you aren't fighting with trying to make one driver produce the entire spectrum, and the woofer and tweeter should be more efficient at their jobs, making the overall output louder. The SM and BB2 have that advantage. The BB2 woofer gets freed up to produce more extended lows (aided by the bigger cab size), or the SM is capable of getting as loud as the SC (though not necessarily sounding the same).