If you look at the original plan for the E12 prior to feedback, it was to use 6x AAA batteries. I think having a single battery with the current charging scheme is an improvement in usability over that... The original plan was to have higher output levels and performance than the E11, and that's generally not happening without a larger size.
Anyway, if you're using IEMs on the go with an amp strapped to a phone, you don't want an E12 anyway. There are other amps out there for that. That said, obviously a shorter length would be better, but at the expense of what? There's also something of a trend towards larger smartphones.
It uses LME49710 + LME49600 design running off +/- 11V rails. Output impedance should be low, unless they decide to add some for whatever reason. However, that seems unlikely considering the stated output power levels below and the rails used.
See a previous post:
1.3W @ 32 ohms is about 6.5V rms (18.5 peak-to-peak, not far from 22V to work with). Around 6.5V or a little more can drive 600 ohms Beyerdynamics and so on very loud, same for planar magnetics like HE-500 / LCD-2/3, etc. To be honest, E9 caps out at just above 7V and is quoted at 1W for 16 ohms. This thing can apparently do E9 levels of power, yet it can run off battery.