What's the difference between a small bottom camera and a full-frame camera?

The core difference is the size of the sensor. You can simply understand that the small bottom camera is part of the center of the picture taken by the large bottom camera. So if you don't enlarge it, the picture quality level is equivalent. If you zoom in, it is difficult for a small camera to have ideal image quality because the original imaging area is small. This is one of the reasons why in the digital age, professional photography is still dominated by big-bottom cameras (even larger ones in Quan Huafu).

As for the difference of image quality when the light is weak, this is because the size of a single pixel of a small camera is obviously smaller than that of a large camera in order to ensure a sufficient or even high number of pixels, and the single pixel is too small, resulting in too little effective information that can be recorded, because in the case of unsatisfactory light environment, recording errors are easy to occur, resulting in insufficient picture purity. In other words, if a small camera with a very low pixel value is compared with a large camera with a very high pixel value, under appropriate conditions, it is possible that the picture purity is equivalent because the difference between individual pixels is small.