Here you have a view of the external battery of Energizer (XP18000A) of 18 000 mAh.
Energizer allows DC 19V, 3.5A in output : 19*3.5 = 67W
There are 2 batteries, so 2*3.7 = 7.4V then batteries allow almost 18A ... (9A for each battery)
The boost converter doesn't have heat sink !
How is it possible ?! I found this picture on a russian website so I can to read components.
Moreover, it contains regulator of li polymer/ion charge, 5v - 16v - 19v converter in the small PCB.