The size of the brain is relative to the body. Assumig. Intelligence simply by overall size is foolish. Larger. Odors need larger brains to process all the information their body is sending them.
Vision for example, is a good example of this. Very little of your eye is actually providing high definition vision.. if you were to double the size of the part of your eye that provided such vision your brain would need to be noticeably larger to just be able to process that extra information.
General rule I have seen is brain size relative to body size, tends to correlate with intelligence. Brain size/body size, the higher that number, genrally the more intelligent the animal appears to be
But neanderthals didn’t have bigger bodies. They were actually slightly shorter, although stockier
They had larger brains, and at least based on the skull shape, appeared to have more visual processing and perhaps more motor cortex function. Unfortunately, they probably had less prefrontal lobe, which I think is why they basically went extinct. Those clever, cunning, and more complex strategizing homo sapiens couldn’t have these rustic brutes competing for their game and made it a point to root them out.