Who says there is a limit? Nature evolves for the environment you are in, an example of this is recent studies suggest some birds like ravens (that can make tools to get food) and some parrots that can mimic human voice are just as intelligent as monkeys, but we can not measure the intelligence in the same way as you would a bird vs monkey.
Nature has developed intelligence based on what the creature needs to survive.
With brain to body mass our current interpretation may be wrong as bird brains are developed differently to our human and even monkey brains but they can reason.
Ravens have been seen to drop nuts on pedestrian crossings and waiting for the cars to smash the hard nut shells and when the cars stop moving the birds swoop down and get the nuts.
Ravens have been observed leading wolves to injured animals so the wolves can kill the animal and they both share the food.