ExpertReviews has published some hands-on photos of the Energizer Power Max P18K Pop and its ample 18,000 mAh battery. The smartphone, which Energizer has been displaying at MWC19, offers reasonable hardware on top of the stout battery that can also be used as a power bank. The P18K Pop has been priced at €600 (~US$681).
https://www.notebookcheck.net/Hands-on-photos-of-the-Energizer-Power-Max-P18K-Pop-show-the-portly-18-000-mAh-battery.410811.0.html
It still presents an interesting talking point, product design wise. Really chunky of course but I roughly estimate that the motorized popup cam module are a large factor.
Which brings us to the next thing; surely anywhere above 4000mAh to 9000mAh range maximum are more than doable for most OEMs and could easily be HALF this phone's size.