For example, we can assume that the average house cat is equally intelligent as the average tiger. The average tiger is about 100 times heavier than the average house cat. But its brain is also bigger than the house cat's brain. Since the tiger isn't any more intelligent than the average house cat, why does it need a bigger brain?
Brain size isn't necessarily a determinant of intelligence. That notion was a assumption of neuroscientists of the past, a speculation that lead to many problematic socio-psychological projections.
The size difference of brains might simply match the morphology of the species, but sometimes it doesn't scale so evenly, as with crocodiles, whose brains remain roughly the same size their whole lives even as their bodies grow. I personally wonder if this relates to the hypothesis that brains evolved to meet the need for movement (the "moving hypothesis").
Often larger animals have larger brains to fit their morphology, but they don't necessarily have more neurons. Having more neurons might be the reason for the greater intelligence (or, perhaps a better phrasing, greater cognitive flexibility) of species like Homo sapiens. This hypothesis is explained in detail in Suzana Herculano-Houzel's paper "Brains matter, bodies maybe not: The case for examining neuron numbers irrespective of body size" http://www.suzanaherculanohouzel.com/herculano-houzel-2011-anyas/