It’s the ratio that’s important, not absolute size. Elephants have enormous brains because every square inch of skin, muscle etc etc has X number of nerve cells that the brain needs to communicate with. If there was a hominid close to our size, but with a substantially larger brain, it would be indicative of intelligence.
But again, we have no record of exactly what those cavities were filled with. They might have been more stupid than Alfred E. Neumann and yet capable of more prestidigitation than a Liberace, and we’d never know it because all we got are bones.