How is that different than the income bonus though? If you have 5 cities with 12 mines each, adding another city (and 12 mines) increases your coal output by 20%, whereas your loss in production bonus is only 10%. Going from 9 cities to 10 cities would be 108 mines to 120 mines, an 11% increase, more than the 10% loss of an additional city, etc.
It's exactly the same as the increase increase/decrease trade-off of adding a new city.