PSA: Only the Pixel XL gets 18W charging, but it might not matter
Pixel XL does charge at 18W and Google has updated the spec sheet for the smaller Pixel, but hasn't commented on why there is a difference in chargers. And, the difference might not make much difference in terms of charging time. The 18W charger for the Pixel XL may charge the device about 20% faster if it were to be constantly charging at peak speeds (which is rare anyway), but because the battery is also 20% bigger than the Pixel, the difference washes away.Google has admitted that Nathan is correct. The Google
The reigning theory is that Google had concerns about heat while the device was charging and so limits how much power it pulls. According to Nathan K, you should be okay even if you use a difference charger, because the software of the Pixel "prefers" 5 volt/15 watt, but can and will handle 9v/18W "when absolutely forced."