By increasing the size of the piezoelectric ceramic disc used in ultrasonic humidifier, does it increase the amount of mist generated?
Note that "large volume" mist generators don't have large disks.  Instead they use multiple disks.  

Note that mist-generators use ~1MHz ultrasound, while ultrasonic cleaners with 3in disks at 30KHz only create droplets and no mist.  PROBABLY the disk size/frequency is important, and a large disk will give low frequency resonance, and different water interaction, and only create droplets or fine spray.

At work we use Lithium Niobate piezos above 10MHz generating micron-scale (invisible) droplets for mass-spectrometer sample input.  Much higher freq, much smaller mist.  Here's various videos of surface-wave nebulizers, producing audio at radio-freqs.    Higher frequency gives smaller ripples on water, which then spit out smaller droplets (like "chop" in ocean waves.)