I assume Germany has a modern program of mine rehabilitation, so you're only able to see the parts that are currently part of the active mine. The historic mine footprint of an old mine is probably twice or thrice what is visible; much larger but invisible from satellite because it has been rehabilitated.
Eg, the area just north-west of Tagebau Hambach is probably mine rehab; you can tell because it is a deeper green than the surrounding farmland and if you look closely the massive contours of the waste emplacement areas might be visible.