The rear camera of the Samsung Galaxy S8 phone features a 1/2.55" sensor with 1.4-micron pixels. If you are a tech savvy, you probably already know that larger pixels contribute to better image quality and better low-light performance. The S8 camera sensor is relatively large, it's off 1/2.55-type. What's great about this sensor is that instead of going with a 16MP resolution, it features "only" 12-megapixel resolution, thus enjoying larger pixels. Most phones have around 1.2-micro pixels some even go lower with 1.12-micron pixels. The larger the pixels the better. Larger pixels can hold more photons of light. In other words, the sensor can hold more color data for each photosite.
Also having 12MP resolution isn't a bad thing. Some people might think that having a 16MP makes a camera better, but this is definitely isn't the case. 12MP is a very good resolution that keeps a good balance between image resolution (image details) and low-light performance. Small pixels have negative impact on the image quality and low light performance, even the high end sensors from Sony suffer from that. You can't beat the physics, at least not using today's latest available technologies in the consumer market. Sensors with smaller pixels will produce significantly more image noise and will produce unimpressive results in low-light conditions.
The Samsung Galaxy S8 could have performed well in low-light maybe even with smaller pixels due to its very fast f/1.7 aperture lens. The thing is that Samsung isn't aiming for an above average performance with the S8 and S8+, it's aiming to be a top performer. So the combination of a 1.4µm pixel size, with a 1/2.55" sensor, OIS (Optical Image Stabilization) and fast F1.7 aperture lens; all provide the camera with better light-gathering capabilities to capture stunning images, even in low-light situations. It's all about component tuning, image processing and system optimization's that can help produce even better looking image. I expect nothing else from Samsung.