I may be being naive here, but doesn't SPL always equal SPL?
I see this and similar questions are quite frequent, so this must be something that many are curious about. And you should be.
If you open up a book about acoustics, the first thing you see is the wave equation - a differential equation describing how sound travels through air. For most us, this equation does not make much sense, since we do not have a solid mathematical background that enables us to understand how this relates to sound and how this works.
What we can see, is that sound is characterized by a pressure amplitude, and a velocity vector with direction, amplitude and phase related to the pressure. Measuring pressure only does not give a complete representation of the sound.
How this works, and how it relates to sound reproduction - especially in the bass range - has been discussed for several years now, but it is not well known outside some of the more obscure parts of the hifi-world.
Pants and clothes flapping happens when velocity is high enough, at quite low frequency. The mechanical impedance of clothing in air is such that it does not require high intensity (intensity == acoustic power), which again means phase and pressure amplitude is not important. To create such a soundfield, has more to do with room acoustic properties and placement of observer (the flapping clothes) and sound source. So a 5" can do just as good as 4x 18". Provided the necessary acosutic output from the source is present, and this is where size matters - a small speaker simply can not output enough sound pressure at low frequencies.
Close to an acoustic radiator, the sound field changes. So that the sound field can be affected both by size and principle. Close to a ported box, the velocity is much larger compared to a sealed box, and a horn with larger radiating area, will maintain this effect at a greater distance. This can be measured, it is not something obscure and mystical, it is very real.
At the listening position, there will usually be no measurable difference in velocity close to the floor, between say a small horn and a sealed system.