Diet has long been hypothesized as a driver of change among hominins, especially with regard to the increase in brain size. Dietary niches reconstructed based on these fossils showed that the Australopithecus individuals had diets very similar to both contemporaneous and modern herbivores but different from carnivores.
Brain size is not strongly correlated with intelligence. Crows and octopi are considered to have similar intelligence as a human child, despite having much smaller brains. Whale brains are much larger than ours, but they’re not considered especially intelligent.