[Nexus 6P Q&A, Help & Troubleshooting] cb474:
Are you sure you need all that resolution? Even at 9.6 MP that's more information than could be displayed on a 4K television screen.

Keep in mind that although the IMX377 is not as high resolution sensor as some of the 16 MP and 20 MP sensors seen these days, it is a very large 1/2.3 sensor (large for a phone). So it is actually physically the largest sensor that's been put in any flagship, except the Xperia Zx phones and the great Nokia phones (like the 808, 1020, N8 that have sensors which remain years beyond anyone, even though they are no longer new phones).

So the reason the 6P has "only" 12.3 MP is not because it is a "low" or "small" sensor. It's a very large sensor (again, for a phone). But instead of using the extra size for more pixels, which is a waste for almost all purposes, they use the extra size for larger pixels. This means the images should have better color accuracy (lack of reviews of the phone proving this, notwithstanding), because the larger pixels can catch more light. The overall larger sensor also improves low light performance and noise.

Even the Z5, the only other phone with a 1/2.3 sensors, doesn't use it's full 23 MP to create unnecessarily enormous photos with details no one needs. Instead its default setting to to produce 8 MP images. The extra pixels are used to oversample, using many pixels to composite into one effectively larger pixel, for the advantages of color accuracy and lower noise. This is the "Pureview" technique pioneered by Nokia with the 808 and its huge 1/1.2 sensor, that's actually 41 MP, but defaults to "only" 5 MP final images.

So unless you have a very specialized need for more detail than the already massive amoung of detail of 9.6 MP or 12.3 MP (and assuming you even have a screen capable of displaying more than the 8 MP it takes to fll a 4K screen), the benefits of smaller images, with larger pixels, and a larger 1/2.3 sensor far outweigh pointless extra pixels that people get obsessed with but serves no purpose for most people. Remember, the megapixel race in phones and consumer cameras has always been a mistake, that produces a level of detail people don't need and can't see, that produces unnecessarily large file sizes, and that actually creates problems like reduced color accuracy and increased noise. Physical sensor size matters a lot, megapixels, beyond about 5 MP or 8 MP serve no purpose (unless you're creating billboards or printing wall sized photos).

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One side note, if your current phone is the LG G4, as your signature suggests, I would not expect to get better images out of the 6P. The G4 has arguably the best camera in a current smartphone. It already has a pretty large 1/2.6 sensor. Even though it's pretty high resolution at 16 MP, apparently they've done something to really reduce the amount of noise you'd normally expect from a 16 MP sensor. And it has one very large advantage over the 6P, which is OIS (optical image stabilization). I'd really wait for reviews that compare the G4 camera to the 6P, if this is important to you. At best, I'd expect the 6P to be as good as the G4 (and it may not be), but I'd be surprised if it surpassed the G4 for still images. And without OIS, there's no way videos will be as stable on the 6P, whatever other merits it may have.