Here is a perfect mod for auto-miners. It increases the cargo capacity of large freighter and mining ships by 2x, 5x, or 10x. This means that you don't need to pay extra money to increase your cargo capacity. Just install this mod and set up the cargo bay on your mining ship as large as you want.
Since this is a simple edit and not an overhaul mod it will not affect your game in any way, so it is completely safe to use.