as far as i understand it, okv hits on the major points.
to understand the issue, an understanding of how drivers work will help and an understanding of the spectral content and dynamic range of the material is also required.
first, the driver.
here is a picture of a speaker.
the voice coil sits in between the top plate and the pole piece (which runs down the center of the speaker and connects to the back plate).
as the field is energized the cone begins to move.
as the voice coil moves away from its center position, the "BL field" changes. B for magnetic field, L for length of wire in the coil.
to move a lot of air, either the speaker must move a lot back and forth AND/OR the speaker must have a large cone to move a lot of air even with a small amount of linear travel.
so there are two ways to do it, either a small cone that moves back and forth a great deal or a large cone that moves back and forth a little.
an 8" driver has to move about 10.0 mm in order to produce 106db @1m@60hz.
a 15" driver has to move about 2.4 mm in order to produce the same 106db @1m@60hz.
xmax is typically specified as the point where the bl field has dropped to 70% of the value with the driver sitting at rest at position 0.
if both of these drivers have an xmax of 8mm, the 8" will be way out past its xmax and its force will be dropping off, while the 15" will be well within its xmax and its force will remain strong.
now the second topic is dynamic range of music. the loudest part of the "kicks & crescendos" vs. the average level of the content. for well recorded music, that difference can be 20db or more. so now if you are listening to music at a very loud average SPL, say 90-100db, the peaks in the music could be 110-120db or more.
obviously, if you are listening to loud, well recorded, music with 120db peaks, the 8" driver simply can't do it. as we already saw, it is running out of gas even before reaching 106db, so 120db is completely out of the question.
the third topic is spectral content of music. because the ways our ears/brains work, we are much more sensitive to sounds in the 300-2000hz region than we are too the 50-150hz region where the kick drums/synths/bass guitars et al have fundamentals. see "equal loudness curves" for more on the topic. suffice to say that most music has 10db or so of eq built in to the low end to account for this. if it didn't drums would sound weak and music would lack power. you don't have to do anything with your tone controls, it is already baked in during recording/mastering. what this means though is that for an average level of "loudness", we need an additional 10db of spl just for mid-bass. so now instead of 120db, we are looking at systems capable of 130db. if that sounds like a lot, it is. rock concert level tends to be up in the 120's depending how close you are to the speakers.
therefore if you want truly uncompressed, full-level, loud, music try to get a system that is capable of 120-130db with drivers moving about 1/2 their xmax or so. that will allow for full force all the way up to the volume required for the "kicks and crescendos".
how many speakers is required to do this? a pair of really good pro-audio 15" drivers might have xmax ratings around 8mm. at about 5.5 mm of travel, a PAIR of 15's will hit 120db @60hz. the single poor little 8" driver would have to move almost 48 mm to produce the same amount of sound...and of course, that would be the end of that driver!
it is about that simple. almost all of the confusion surrounding this topic comes from manufacturers who try to convince you that a single waf-friendly 8" driver is all you will ever never need and/or is "more than sufficient" when in fact the truth is...it ain't even close.**
power compression, distortion, inductance, phase, air-coupling, air-velocity...there is more to the story, but it is mostly just hitting the target spl in the linear range of the bl field in the driver. get that and your most of the way there...
** now I'm not saying that everybody needs full rock concert spl. that is a personal choice. the point is simply what is required to get there for those who want it.