I think given that humans evolved to make trade-offs in order to have large brains is a sign that humans generally having large brains is useful, but the usual idea that a large brain is strictly necessary for human intelligence doesn't necessarily follow from that. It could be that what we think of as human intelligence doesn't need all that much brain matter to exist inside of, but having more brain matter increases redundancy and the chance of survival if you starve or get some brain damage, adds potential capacity that isn't always necessary, or it increases the chance of usual human thought processes developing somewhere within the brain to begin with. Maybe having more brain matter is more important as a very young child in order to help the brain bootstrap itself, but then after that most brain matter is redundant.