Brain mass scales with body mass at less than a one-to-one rate across mammals, so smaller species end up with brains that look disproportionately large as a simple fraction. A squirrel monkey weighing under a kilogram carries a brain of roughly twenty-five grams, giving a ratio above the human figure. This is a scaling artefact, not evidence that squirrel monkeys outthink people.
The measure that controls for that scaling is the encephalisation quotient, which compares actual brain size against what a mammal of that body mass would be predicted to have. On that measure squirrel monkeys score well above the mammalian average and among the higher New World monkeys, while humans score far higher than any other species.
The traits that matter for their ecology are more specific: squirrel monkeys forage in large troops for cryptic insect prey, and troop sizes of several dozen animals are ordinary. Females of many populations have colour vision that varies between individuals, so some see in three colours and others in two.