The 4-megapixel front-facing camera uses a sensor with a single pixel size of 2μm (micron), which is almost twice the size of a typical front-facing camera.
A single pixel has a larger area of light, which means it can receive more light and produce better images. Increasing the size of individual pixels results in a more pronounced quality improvement than increasing the number of pixels alone, resulting in less noise, overall brightness and more detail in low light.
Notes:
The pixel with large photographic area can capture more photons than the pixel with small photographic area. Under the same illumination, the pixel with large photographic area is easier to generate output or more sensitive to the analog-to-digital conversion circuit than the pixel with small photographic area. In this way, the camera with large pixels can still take bright pictures with less noise and rich details in a dark environment.