Neurons are tiny. The capacity for intelligence seems to be linked much more with how they are connected than how large the resulting structure is.

We would only expect to see intelligence evolve in an organism to the degree that it improves their fitness. Intelligence is not a universally good trait; it is expensive to maintain.

Koalas are drop dead stupid because they’ve gotten themselves stuck in a valley on the fitness surface, not benefitting from intelligence. Ants don’t need to be individual geniuses, because the colony’s intelligence is an emergent property arising from many much less complicated little nodes.