There's not a lot to do in space, so bored people in close quarters resort to what they always do, and have large families as a result. The small families that are the norm in today's developed world are a historical anomaly.
In rural China, it was nearly impossible to police a "one child" rule until they became more economically developed, because rural people had nothing to take away. If evicted from one bush, they just sleep under another one. Jail meant more and better food, and better shelter.
Your miners are in a similar position, jail time trades hard dangerous work with some liberty for easy living and guaranteed meals and marginally less liberty. The government can't take anything away without it costing them (the government) more than they gain.