@YaFreekin Right : you are correct and I edited my post to remove the misleading part which was not well formulated and indeed not the reason for the difference I see in practice. I think I just tried to precise that we don't get twice the mAh, just twice the voltage, not that it really matters.
There are some losses in the step down regulator (advertised at 93% efficiency) but they don't account for much.
The main reason I only get 4 bowls per charge (and that is by driving the cells down to 3.4V, lower than with the Mi that ends more at 3.6V) is that my Zion sessions are longer. I never use them at full power so I end up getting more hits that require more preheat and trigger time. The Zion allows to do the temperature stepping in a very organic way, so I end up spending more time extracting at low temp, then mid temp and ending at high temp. Whereas the Mi is always full on and tends to force me to extract faster at higher temp, with shorter sessions.
The Zi also outputs a constant power whereas the Mi peaks at 50-60W only with a fresh cell. The power figure only goes down with the voltage afterwards. So in a sense the Zi outputs more watts continuously and thus depletes the cells faster. This is however partly countered by the fact you need to hold the trigger longer and longer with the Mi as the voltage drops.
There are other differences to take into account, like heater size and IR, larger body mass (heatsink effect) etc. So it's not that simple, but overall the main reason is the pace at which I do my sessions. If the heaters were exactly the same and the Zi was started at full power then gradually decreased, it should be closer to 2x the run time of the Mi.
