According to Byron Sharp, brands grow primarily by targeting new customers and rarely by extending their relationships with existing customers by cross-selling or introducing new uses for their products. Brands grow when they are good at acquiring new customers; they contract when they are not.
Heavy buyers aren't worth targeting because odds are that they have already bought what they need from you. (This Sharp calls the "heavy buyer fallacy.") Meanwhile, light buyers make up a more significant ratio of buyers than might be assumed, and probably contribute more to the bottom line than heavy buyers anyway. Cross-selling, too, is difficult and expensive; if a buyer already needed the product being cross-sold, Sharp says, they'd probably have already bought it.
Sharp, Byron. How Brands Grow: What Marketers Don’t Know. Illustrated edition. Oxford ; New York: Oxford University Press, 2010.