I have 3 dogs that weigh ≈ 5 kg, ≈ 20 kg and ≈ 45 kg. The smallest dogs whole head would fit in the mouth of the biggest dog. Although I haven't measured it exactly but by just looking at the dogs I would expect the weight of their brain to approximately correspond to their body weight, that is the 20 kg dog's brain is 4 times heavier than the smallest dog and the 45 kg dog's brain is 10 times heavier.
Meanwhile, all three dogs seem to have similar intelligence.
What is all the extra weight of the brain of the bigger dogs used for?
You can expand this question and note that an average mouse brain weigh 0,5 g while an elephant weighs around 5 kg. Elephants might be smarter than a mouse but not 10 000 smarter. Besides, the brain of a human is much lighter than an elephant and we obviously are much smarter.
Why do bigger animals need bigger brains than smaller animals to accomplish approximately the same intelligence? Is it something along the lines that they have more muscle fibres and every single musle fibre needs a few neurons to control it?