Hi everyone!

My first post to this forum, so let´s start with a newbie question: How to increase mining income?

When you look at the building descriptions, it says an ordinary mine should give 2000/turn and a mining center/deep mine/gold vein 5000/ turn. However, in city income statistics a mine gives 600 or 800 and a mining center 3000/ turn.

So, is there any way to increase it? Is it related to population?