With mirrorless, you can put the back of the lens closer to the sensor (don't need to flip a mirror out of the way at high speed) which reduces the glass needed

With mirrorless, you can put the back of the lens closer to the sensor (don't need to flip a mirror out of the way at high speed) which reduces the glass needed for the same effective focal length. Less length = less glass. Less glass = less mass, which means less force required for an electric autofocus motor to push around all that glass. Modern zoom lenses have anywhere from 11 to 30 elements, in 3-11 groups, that move at different speeds relative to each other. Doing that with a tiny electric motor, precisely, for a decade is a tough task, especially when the mass of the glass starts to exceed 1kg. With a mirrorless camera, you can reduce the mass of the lens by ~10% for a full frame camera. With an APS-C sensor, mirrorless, you can reduce the mass by 33%. With a "micro four-thirds" sensor, the mass of the lens is less than 50% of a full frame. Rough numbers. As with everything, there are pros and cons to going with smaller sensors than full frame.