Why was it inevitable that they would run out of dog food?

From the text:

In the nature of Arctic travel there was a reason why fourteen dogs should not drag one sled, and that was that one sled could not carry the food for fourteen dogs.

It was inevitable that they should go short on dog-food. But they hastened it by overfeeding, bringing the day nearer when underfeeding would commence. The Outside dogs, whose digestions had not been trained by chronic famine to make the most of little, had voracious appetites. And when, in addition to this, the worn-out huskies pulled weakly, Hal decided that the orthodox ration was too small. He doubled it. And to cap it all, when Mercedes, with tears in her pretty eyes and a quaver in her throat, could not cajole him into giving the dogs still more, she stole from the fish-sacks and fed them slyly.