The iPhone X comes with a larger sensor size (for the rear camera) compared to the iPhone 7. The exact sensor size isn't listed on the official website, but some suggest that the iPhone X comes with a 1/2.0-inch format sensor. I think people just try to calculate the size based on the size of the iPhone 7 sensor and the fact that Apple said that the new sensor captures 83 percent more light. I also don't know whether this is just for the wide-angle lens or for the telephoto one as well. i believe, considering the setups of similar phones, that this is for the wide-angle camera only. The 83 percent more light was mentioned regarding the new iPhone 8 all-new sensor with deeper pixels and new color filter, but this is the same sensor used for the iPhone 8 Plus and iPhone X wide-angle camera as well.
Having the option to capture 83% more light means that you'll be able to capture photos with higher saturation and dynamic range, and low light performance is increased as well. I personally would have preferred to also have a faster aperture like an f/1.6 or f/1.7, same as the competition, but these improvements might showcase a marginally better image quality overall. The second telephoto lens is obviously isn't good for low-light photos due to its slower f/2.8 aperture lens, but the Optical Image Stabilization should help to improve the low-light performance.
I wonder how the new iPhone X cameras will perform in DxOMark image quality tests. It's nice to hear about incremental performance figures but we need to see some objective test results to really see how good the iPhone X cameras are compared to older models and the competition, mainly the Galaxy 8, Note 8, LG V30 and LG G6.
When I saw a 12MP sensor I knew that Apple optimized it for image quality. It didn't went crazy with the pixels and like Google with its Google Pixel phone, it made sure to use a large sensor to improve the image quality by a noticeable margin, but that yet to be seen. By the way, a 1/2.0" is a relatively larger sensor, even larger than the sensor of the Google Pixel camera which is 1/2.3". And again, if it is the case, I don't know why Apple doesn't emphasize this feature and showcase on its own website. It didn't even mention the sensor size in the launch event.