Suppliers’ bargaining power mainly comes from their market position or top supplier status. However, it is also affected by the horizontal competition from top supplier size homogeneity based on the purchasing proportion of a buyer. Using a sample of listed companies in China, we find an inverted U-shaped relationship between the supplier size homogeneity with shared customers and trade credit provisions. As size homogeneity increases, suppliers may increase (decrease) trade credit provision to expand sales (for cash income). That is, based on the degree of supplier size homogeneity, the homogeneity may strengthen or weaken the supplier competition effect. We also find that if upstream and downstream firms have associated relationships, or if the business environment is poor, the competition effect from supplier size homogeneity is not obvious, whereas the weak financing ability of buyer promotes the supplier competition. In addition, the competition effect from supplier size homogeneity is not observed in state-owned firms.