Spot price can be thought of as an 'industrial wholesale' price for the metal. The mints have to be paid to change the metals from their raw form into the products that investors want.
The easiest and cheapest products to make are large bars, like 100 oz. silver bars, or 1 kilo gold bars. The premiums above spot price for the larger products are typically the lowest. The premium above spot price usually rises with the smaller products since there is more work involved in producing them.