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Intelligence doesn't directly vary with size either, that's the scary thing. Look at whales, blue whales, sperm whales, etc. They don't have politics or war.
As a general rule brain size relative to body size is a good predictor of intelligence, although other measures can be better in different lineages. For example, primates have a high neuron density thus absolute brain size is a good predictor of intelligence amongst them. This high neuronal density also means that humans - despite not having the biggest brains - appear to have the most neurons of any animal.