On Sony cameras, there are 2 types of camera categories, A-mount, which uses a mirror and E-mount, which does not use a mirror. Even though its size is very small, Sony E-mount cameras have sensors that are not inferior to A-mount cameras.
Source: https://camera.co.id
The main difference between these two types of lenses is the distance between the rear lens and the camera sensor. The A-mount requires a longer distance from the lens to the sensor because there is a mirror on the inside of the camera. Meanwhile, on E-mount, the distance from the lens to the sensor is shorter because there is no mirror, so the size of the lens is smaller.