The Google Pixel back camera features a large 1/2.3-inch (6.17 x 4.55 mm) camera image sensor, which is relatively large by mobile phone camera standards. The camera has 12MP and because of the relatively large sensor, each pixel size is measured in 1.55-micron pixels.
The Google Pixel sensor size is one important factor that helps promote better image quality. 12.3-megapixel resolution is quite a lot for a mobile phone camera. Google had to use a larger sensor to be able to have larger pixels and for the camera to perform as Google wanted it to perform.
The Pixel camera sensor is said to be the Sony IMX378, an upgraded version over the IMX377 that appeared in the Nexus 5X and Nexus 6P rear cameras.
The front camera utilizes a 1/3.2-inch 8MP Sony IMX179 sensor with 1.4 µm pixel size, which is smaller than the primary camera sensor but has lower resolution, so it still results in very good image quality in general.
The Google Pixel back camera uses the same sensor as the Xiaomi Mi 5S. This sensor is known to be the best camera image sensor one the market right now. The one that was able to beat out the competition, including the Samsung Galaxy S7 edge, iPhone 6, HTC 10 and LG G5 among others. Of course, the sensor itself doesn't tell us the whole story, and the advanced image processing can certainly translate to improve visuals, but when starting with an IMX378 sensor as the base, you can rest assured that the camera would perform admirably - and that's probably why Google has chosen this sensor for its branded high-end smartphone.
I remember how amazed I was after inspecting the Mi 5s sample images on phone arena. I knew back then that we are dealing with a remarkable camera and it will be interesting to see a side by side comparison of the Google Pixel camera versus the Xiaomi Mi 5s to see if there are some prominent differences in image quality and low-light performance.
Overall, the Google Pixel and Pixel XL feature the best camera sensor and the sensor quality was already proven by DxOMark, giving it an overall score of 89.