Human brains are not the largest in the animal kingdom. Are elephants and whales smarter than us? We don't think they are, but we don't really know. It could be that they're much smarter but in different ways, maybe somatosensory or social or other ways we don't understand. It could also be that their brains are less efficient due to less selection pressure for efficiency.

In humans there is only a weak correlation between brain size/mass and IQ or other metrics of intelligence.

Then there's utterly wild stuff like this that reminds us of how little we really understand about brains and intelligence:

https://www.sciencealert.com/a-man-who-lives-without-90-of-h...

The fact that someone can function like this is incredible and indicates that the brain must contain a lot of redundancy, or something even weirder is going on.

Stuff like that is enough to make you wonder if we know anything at all.

Another similar data point is the spooky intelligence of many birds, like crows, who have tiny brains. Flying animals are under extreme selection pressure for efficiency because they need to be small and light, so their brains have gotten very efficient.