The ZX1 also outputs 15mW+15mW, runs Android and has about half the battery time of the ZX2, but also uses a smaller battery due to the much slimmer chasis (Japanese users had calculated it to be under 1000mAH), while the ZX2 gets double the battery life with an
industry person confirming that its battery capacity is double that of the ZX1 at 1880mAh - and it gets roughly double that of the running time of the ZX1. So to get double the battery life such that the device meets the requirement of CEO Kaz's "must last a cross Atlantic flight on one charge", they put a bigger battery of double capacity in there and that's about it, is all pretty straight forward maths/engineering. Of course Sony's not going to tell you "well we didn't increase the power output of the headphone out to meet our CEO's request" because that would be extremely bad PR, but again if you look at the spec it's straight forward maths/engineering.
Now that we know the WM1A/Z outputs 4 times as much power on SE in a body that is roughly the same as the ZX2 - battery tech certainly hasn't improved that much since the release of the ZX2 and we are already able to see the internal of the WM1A/Z already from all the photos and they haven't hollowed out all the components to put in the biggest battery in there, so the reduced power drain must be attributed to the things that did change even in light of the massive increase of power output. Which means it's either the removal of the WiFi radio (unlikely - test conditions almost never turns on WiFi for DAPs) or the OS (very likely, since you can't turn off the OS). So by logical deduction it's going to be the OS, there's almost no other way to get around this conclusion.
Plus it's no secret that Android is a battery hog in the IT world, and pretty much all the DAPs which runs on Android on the market now struggles with more than 10hrs playback time. So it's pretty obvious what is to blame here.