Differences between small vs. large mid driver

Andy, it'd be easy to write a book about this, and I'm 2-finger typer on a damned French keyboard on vacation in Grenoble, so I gotta keep it short...plus I'm not the expert here.
Driver diameter is chosen to balance many needs. Generally, the larger the driver (and motor), the lower the passband, the narrower the dispersion at higher frequencies.
Simply, it gets hard to mate an 8" driver with a 1" tweeter, as there passbands barely intersevt smoothly, and off-axis response usually suffers from excessive "flare".
It follows that the necessary low crossover for such a design risks overpowering the more fragile tweeter's motor.
Rare is the 8+1 that sounds great (I do remember a Genelec pro monitor that was pretty impressive, but again probably intended as a console on-axis monitor rather than normal room use.), but it's a cheap way to get a lot of bass, generally as a trade-off with smooth midrange.
Smaller mids have smoother upper freq response, so usually will result in better blends with tweeters, sometimes allowing the crossover freq to be pushed WAY up, thereby reducing all sorts of crossover response crap in the critical midband. (B&W crosses the Nautili at 4k, but not to my personal liking; Verity Audio uses such a fine mid that they don't cross it until way up at 5500Hz! The "coherence" is phenomenal, as a result...but I digress.)
Since small mids don't work too efficiently down low a designer has to decide whether to boost the bottom with a vented or very large enclosure, or of course simply design the speaker with limited bass. The former may work very well within its dynamic range, the latter may benefit from boundary support or of course a subwoofer. The obvious rejoinder is to add a third driver for bass response, taking the load off the mid, and thus allowing a smaller mid that blends better with the tweeter.
In practice one sees a lot of 5" and 6.5"ish +1" monitors. GENERALLY the larger ones are punchier, having greater dynamic capability, and a more open sound. Many 5" mids sound pinched to me, with a "cupped" coloration. In my own work with an 8+5+0.75 three way I couldn't quite get the lower mids to sound as natural as with my 6.5+1, for example. Of course many designers DO have tremendously successful designs with 4-5" mids, as the physics postulates that size range as most appropriate for the female voice, for example.
Designing ANY three-way design gets much trickier in practice, despite the easy passband summations of 8-10" + 4-6" + 1"...but this is a big subject.
You noticed an inverse correlation between midrange size and price. This is a personal idiosyncracy, and perhaps the result of comparing a well-honed smaller design using a great midrange driver vs a larger design using a cheaper "mid-bass" driver. I dunno. The popularity of 6-7" + 1" two-ways indicates the success of balancing fine, well-balanced response and cost. Since it's a bit easier to improve upon the critical midband with a smaller driver, a designer will (almost) necessarily give up bass response to achieve this holy grail...sometimes the results command a higher price, even though the driver is smaller. There are too many other factors involved, some of which I hope I shed a bit of light on. A bientot. Ern

Good discourse in this thread!

My Coincidents are a GREAT speaker, using an 8" Seas P21 Excel, with a 1.25" ScanSpeak Revelator. The midwoofer's phase plug helps extend the driver's upper frequency performance and address the point Ernie brings up regarding the difficulty in implementing this. The 10" Peerless driver in the integral sub makes sure the speakers go all the way down. There is no crossover between the 8" and the 10", although the 10" does have a cap and a coil to roll it off.

The Fried speakers we're building have the new, outstanding 6.5" Vifa. The Monitors are a two way, and the Studios are a three way, which include the 8" version of this driver. The previous version of the Studios used a Peerless 5.25" midrange (and it's 8" counterpart), but I can assure you this is a MUCH better sounding speaker, in a smaller, more attractive cabinet.

Wait a minute! a "Midrange" driver, by definition lives in a three-way system (at least). The 6 and 7 inch drivers in two-way systems are not midrange drivers, although they do need to pinch-hit for the missing driver. Even the best 7-inch drivers develop ripples in their response starting around 1500 Hz, and tweeter performance is compromised if you cross it over that low.

A midrange driver covers the range from about 600 Hz to 3000 Hz.

El, 600Hz? That's over an octave above "middle" C on the piano! Most mids that cross that high have coherence problems, especially obvious with the human voice. And asking a "big" woofer (8+) to work cleanly at least an octave past there is pretty risky too....
I'm a fan of running a mid "naked", crossing acoustically at
150-200 Hz. Does rule out very small ones, of course. 5.25-6.5 is the generalized best starting point. Cone breakup issues at the top of the passband are minimiwed by use of outstanding cone design/material and a proper motor. Great midrange drivers are usually not cheap, although it's clear that many of them can be made to work pretty well at least for a portion of the 200-4k band (what I call the mids: an octave above the top of the piano).

Eldartford, you are correct in the classic definition of a midrange driver - your frequency range excepted. However, you must allow that a lot of great two way speakers, such as the Merlins, have fabulous midrange, despite "missing a driver" as you say. A midrange should not go down to 600 Hz, but much below that, I would say it should do 100 Hz, as the crossover between midrange and woofer is ideally around 200 - 400 Hz, and there will be a lot of output below the crossover frequency as a matter of course.

Your assertion that the best 7" drivers develop ripples starting at 1500 puts up a red flag in this thread that many a 4" driver also shows. In my experience, the 18 cm (7"+) ScanSpeak drivers are fine out to 3000 Hz. I personally would set the crossover point a bit under that, closer to the 2500 Hz area, but being a big believer in first order crossovers, there is going to still be a heck of a lot of output at 3000 Hz and more.

My favorite midrange drivers are 6.5", which we're using in the new Frieds. Again, my Coincident Digital Masters are some of the finest speakers I have come across (which is why I own them - they are a lifetime purchase), and they use an 8" midwoofer, crossed over in the 2250 Hz neighborhood. As they were also intended to be used on their own, without the matching Troubass subwoofer which they use as a stand, they go just about all the way down. After I rebuilt the Seas P21 Excels, they have some of the finest midrange I have come across.