The Sony Xperia XZ1 comes with a 19MP 1/2.3" Exmor RS stacked sensor. Obviously 19MP is way too much for a mobile phone camera, but the good news is that Sony used a 1/2.3"sensor, which in mobile phone terms it is considered to be large. This translates to a 1.22 micron pixel size, which is not bad considering its very high resolution. This is one of Sony's latest sensor. As of the time of writing I don't know the exact model of this sensor, but it comes with it's one of Sony's memory-stacked sensors, and because it's a new technology, I can just assume that it's a relatively new sensor.
So having a large 1/2.3-inch sensor is nice, but we can't just not ignore the possibility what happened if Sony has decided to use like 12MP with this relatively large sensor, how the image quality could have been. Having said that, this is certainly one of advantages of the Sony Xperia Xz1 camera.