Abstract We exploit heterogeneity in decreasing returns to scale (DRS) parameters across mutual funds to analyze the importance of scalability for investors’ capital allocation decisions. We find strong evidence that steeper DRS attenuate flow sensitivity to performance. We calibrate a rational model of active fund management and show that a large fraction of cross-sectional variation in assets-under-management is due to investors anticipating the effects of scale on return performance. We conclude that DRS play a key role in achieving equilibrium in the intermediated investment management market.