Any Camera Depends On Many factors

The Megapixel myth has treated camera manufactures well over the years, those very increasing, and often meaningless.

Camera quality depends on many factors . Such as -

1) Sensor Size
2) Pixels
3) Aperture
4) Post processing 

While the duel camera system does not offers good quality than a single setup camera 

As Google Pixels have single camera setup while many other brands use double ,triple camera setup ,But Image quality of google pixel is far better than other double or triple camera setup.

** Post processing plays a big role in this result.

How They Play roles in it-

1) Sensor size

The Size of sensor that a camera has ultimately determines how much light it uses to create an image. In very simple terms,image sensor consist of millions of light-sensitive spots called photo-sites which are used to record information about what is seen through the lens. Bigger sensor can gain more information than a smaller one and produce better image.

2) Pixels

In Simple words.Pixel plays role ,at the time of zooming .

3) Apertures

An aperture is a hole or an opening through which light travels. More specifically,the aperture and focal length of an optical system determines the cone angle of a bundle of rays that comes to a focus in the image plan.
In simple Language An optical system typically has many opening or structures that limit the ray bundles and set the intensity of light according to image . Automatically or Manual.

4) Post processing

This process plays role after all these steps .
Camera software interface play this POST PROCESSING process.

Google Post processing is best because, with single camera setup Google Give best Blur Images which knows as Portrait Mode