Hi Dave,
I am not sure what they are doing, but it may be more than the size of the driver, which measures 10". They also use some other engineering trickery... here's some marketing language ...
"The center cylinder (phase plug) in a Zu 10” driver
A machined phase plug assembly, which is not in motion, directly connect to the frame/motor assembly, with both visible and hidden features allowing for the following benefits:
Extending high frequency limits of the driver assembly through a combination of whizzer cone and center pole detail profile and acoustic impedance matching.
MLS diffuser function of pole piece profile and rings aids in the widening of the high frequency (polar plot or beam pattern).
Aids in control of dynamic behavior through diamagnetic shunting (shorting) of spurious B fields.
Zu’s implementation of shunting is regulated, providing virtually no dynamic damping at low level play and increasing logarithmically to maximum in high SPL play.
Dramatic improvement in cooling as the assembly is press fit and uses thermally conductive adhesive. (Only significantly in play when you are pushing huge power, but there nonetheless.)"
and...
"Zu-Griewe driver/box/acoustic loading technology, developed by Sean Casey and the late Ron Griewe, allows improved driver-to-room coupling, fidelity, and bandwidth.
Let us introduce you to Zu’s driver/box/acoustic impedance matching technology. Zu has designed a speaker-driver/box/room loading technology that reduces the acoustic impedance ratio of loudspeaker cone to room, thereby increasing efficiency and reducing cone motion. This original technology significantly widens the usable bandwidth and reduces distortion. It does not introduce distortions common to horn-loaded speakers and is operable through several octaves."
Here's some "white paper stuff"/ marketing:
https://www.zuaudio.com/questions-list/2013/8/18/what-is-a-full-range-driver
https://www.zuaudio.com/questions-list/2013/8/18/tell-me-more-about-zus-10-full-range-driver
https://www.zuaudio.com/questions-list/2013/8/18/what-makes-zu-loudspeakers-so-good