[Baeocystin]

>It's not clear why a mammoth or other elephant-like creature would have such large brain (the African elephant's brain is about 3 times as large as a human's). Perhaps there's some redundancy in the tissue, or legacy regions that are not used today.

This, at least, I can answer, using numbers from https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4053853/ . The elephant brain is massive, but 97%+ of those neurons are in the cerebellum, and is likely related to their somewhat unique use of a trunk. And while their cerebral cortex is twice as massive as a humans', it only contains one-third the neurons.