Help wiring DC motor
You have the relay contact connections shown wrong in the diagram. Since it works, it must be wired right. In the switch position shown, both motor terminals are connected to negative. In the opposite position, both terminals are connected to positive. The motor fuse should be a slow blowing type to accommodate the motor starting current.

The relay coil wires can be small, but the contacts need to be #10 at least. Be sure the relays are DC rated and motor rated.