[wongarsu]

Both absolute size and relative size are poor approximations, but relative size gets closer.

Large animals aren't just scaled up versions of smaller animals. Large animals use the space for more complicated digestive tracts. Their larger appendices allow them to have more joints with more degrees of freedom, and longer appendices and higher speeds require a higher resolution from their eyes to perceive sufficient detail. Skin with more surface area also means many more cells for sensing heat, pressure, damage, etc across the entire skin.

All of this means the brain of a larger creature needs more "IO ports", more neurons to compute sensible outputs for them even for routine tasks, and more neurons to break the wealth of data into a form usable by a cognitive process or an instincutal reaction (mapping to area of body, computing averages over time and over multiple inputs, correlating different types of senses, etc).

All of these differences have different scaling factors. The amount of neurons to count to 5 stays the same, the amount of neurons to process skin sensations scales approximately with skin area, or the square of animal diameter, but the amount of neurons to control motor functions or the digestive tract can grow much faster. Overall, putting brain size in relation to animal size or mass is a decent first approximation and works in practice.