This paper quantifies the origins of firm size heterogeneity when firms are intercon- nected
in a production network. Using the universe of buyer-supplier relationships in Belgium, the
paper develops a set of stylized facts that motivate a model in which firms buy inputs from
upstream suppliers and sell to downstream buyers and final demand. Larger firm size can
come from high production capability, more or better buyers and suppliers, and/or better
matches between buyers and suppliers. Downstream factors explain the vast majority of
firm size heterogeneity. Firms with higher production capability have greater market shares
among their customers, but also higher input costs and fewer customers. As a result, high
production capability firms have lower sales unconditionally and higher sales conditional
on their input prices. Counterfactual analysis suggests that the production network
accounts for more than half of firm size dispersion. Taken together, our results suggest
that multiple firm attributes underpin their success or failure, and that models with only one
source of firm heterogeneity fail to capture the majority of firm size dispersion.