Size of Midrange Drivers

Why, in this day of super materials, do designers still use
mini midrange drivers?

a midrange driver will start beaming (sound thru it will be very directional pretty much like a focused light beam) at a certain frequency.
The bigger the diameter of the midrange, the lower this frequency of beaming.
For example, when I used to own B&W DM604S2 they had a 7" midrange & the frequency that the Kevlar mid would beam was in the region of 1800-1850Hz.
It was distinctly audible - when I would listen to Frank Sinatra (whose voice is deeper) the vocals would be in line with the speakers. When I would listen to Diana Krall (whose voice is higher pitched), the vocals would pop out in front of the speakers while the other instruments would stay back in-line with the speakers.
I had a long discussion in the AudioAsylum with other B&W owners & found that they were experiencing the same thing.

A large diameter mid is not a good thing - the mid needs to be flat frequency response beyond both the lower & upper x-over frequencies. And, only the electrical x-over ckt should be rolling off the mids i.e. the mechanical failure of the mid should not be rolling off the mids (in addition to the electrical x-over). If this does happen, the x-over response of the speaker will be highly uncontrolled & will be signal dependent. Signal dependent behavior of any electrical or electro-mechanical system is a very bad design. The speaker will have a lot of phase distortion - which means the speaker will impose its distortion onto the music signal & will destroy the timbral & tonal nature of the music. Listening to music thru such a speaker will be a complete displeasure (atleast for me).

8" drivers typically cover bass and midrange to some extent in most 2-way designs I have seen.

3 ways use a dedicated midrange and do not handle lower bass frequencies so size is typically smaller accordingly.

I am not sure that there is really any sound reason why midrange drivers should generally be any larger than they are. You have to trust the designer to make the right decisions in each case.

So not sure 8" midrange size alone really matters that much for mid-range. At least for home applications. Maybe for pro applications in large venues where overall size tends to matter more.

Depends how big is your room and how loud you want to be.
8" or higher midrange driver can push significant amount of air and on the midrange frequencies it's quite a lot to fill with sound small auditorium sized rooms.