Why does the cranium of australopithecus afarensis smaller than the cranium of homo erectus?

The smaller cranium of Australopithecus afarensis compared to Homo erectus may be due to differences in brain size and development. Australopithecus had a smaller brain capacity and may have had a less complex brain structure than Homo erectus. The differences in cranial size and shape could also be attributed to evolutionary adaptations and changes in diet, behavior, and habitat between the two species.