Now I don't know a whole lot about cameras... but from what I understand there's multiple things that come into play here, and not just one spec that makes or breaks the camera quality. There's four things that I understand that come into play they are...
1. Pixel count, like 12.3 megapixel
2. Aperture size, like f/2.0
3. Sensor size, like 1/2.3
4. Pixel size, like 1.55u
All of these things are working against each other, what I mean by that is if you increase any of these specs the physical camera size is going to increase. So in order to keep the camera size small enough to fit a smartphone, if you increase one spect you'll have to decrease another spec.
From what I understand the Aperture, Sensor Size, and Pixel Size, all help low light capability. The Pixel and the Samsung have about the same size sensor. But the Samsung has a better Aperture... whereas the Pixel has a better Pixel Size, which helps it offset the aperture. Maybe that's why they call the Pixel the Pixel... cuz it's got big pixels? Lol
I'm far from a camera expert so I could be way off here... If anybody else wants to chime in and correct me feel free.