Corporate managers and executive compensation in many industries place significant emphasis on measures of firm size, such as sales revenue or market share. Such objectives have an important—yet thus far unquantified—impact on market performance. With n symmetric firms, equilibrium welfare losses are of order 1/n4, and thus vanish extremely quickly. Welfare losses are less than 5% for many empirically relevant market structures, despite significant firm asymmetry and industry concentration. They can be estimated using only basic information on market shares. These results also apply to oligopsonistic competition (e.g., for retail bank deposits) and strategic forward trading (e.g., in restructured electricity markets).