Supplier selections are complex but nonetheless strategically important decisions that are influenced by numerous factors. Drawing on the resource‐based and relational view of the firm, we investigate how suppliers’ economies of scale influence the buyer's selection decision, and we illustrate how the influence of scale is contingent upon important economic, buyer, and relationship characteristics. We test the model with a large secondary dataset of actual supplier selection decisions from the automotive industry and show that economies of scale have a strongly positive but diminishing effect on the buying firm's supplier selection decision. These effects are reinforced or extenuated by economic, buyer, and relationship characteristics, with characteristics that are more specific to the buyer‐supplier situation (e.g., relationship duration and power balance) having a stronger moderating effect than do characteristics that are more global (e.g., economic cycle). Our research helps suppliers to better understand how to manage selection probabilities with buyers and provides buying firms with a better understanding of how contextual factors affect the benefit of supplier‐provided economies of scale.