Why are the heating elements not as big as the cushion itself?

For the best heating time-battery-efficiency ratio; we could make the heating elements as big as the cushion/blanket, however a lot of warmth would be wasted. The warmth does not radiate, and you only feel it when leaning against it or holding it against your body due to the infrared technology. If the pad was bigger, it would cost more energy and lead to less heating time.