More broadly, the “big mother” hypothesis, outlined by the biologist Katherine Ralls in the 1970s, suggests that large females might have bigger young, which are more likely to survive, and provide better nourishment and care. If that favors selection for larger females, and sexual competition favors selection for larger males, the result might be both sexes being roughly the same size, Dr. Tombak said. “You have these two forces that are kind of pushing body size up on both sides,” she said.