The point of this futures market is to hedge against price changes that potential mean a bigger cost of a planned trade.
That's problematic since traders want to be sure of a price to pay and to get.
So most of those futures contracts, aren't delivered in the underlying (here silver) but in dollars.
Those contracts, are the hedge "mirrors" of physical trades. The latter is delivered in the commodity, the former in dollars.
Ex some1 needs to buy 5000 ounces and pay a price 5000 x 20 = 100000.
But between that wish/order and delivery, the chance is there that the price increases to for ex $21, meaning he would have to pay 105000. In order to be sure of the price, the buyer takes 1 long futures position. And the seller takes 1 short futures position.
If the price would rise to $21, the futures positions account of the buyer will have accumulated 5000 dollars.
And the one of the seller will have lost 5000 dollars.
So the buyer will pay $105000 but his expense will have been $105000 - 5000 = the intended $100000.
And the seller will receive $105000 but will lose the windfall $5000 and receive the intended $100000.
The ounces represented by the futures positions, are just irrelevant, it's all bout the compensating dollars.