Why were extinct gigantic birds so small?

This review details the six lineages of large flightless birds that evolved in the Late Cretaceous, Tertiary and Quaternary periods of geological time. Estimates of mass for each type of bird suggest that maximal mass is no greater than 500 kg with most species attaining only 250–300 kg or less. By contrast, non-avian Archosaurs of the Mesozoic, and many mammal species of the Tertiary, attained great size with many species reaching several tonnes. Size has been limited in flightless birds because of the strength of the eggshell and in the largest species reproduction was only possible if the smaller males incubated. That reproductive characteristics limit mass in flightless birds suggests that truly gigantic non-avian theropods could not contact incubate their eggs and had to rely on environmental sources of heat energy to drive embryonic development. If fossil evidence ever arises to support proper contact incubation in a non-avian theropod then it is predicted that it will only be from a small (5250 kg) species.