I’m running out of time before work, so I gotta be quick. Apologies.
I’m of the opinion that Spearman’s g is the most important indicator of any sort of intelligence up to about the 95th-99th percentile. A person with a good vocabulary will also be good at math, almost without exception (and these exceptions are always due to brain damage, possibly through malnutrition).
Humans being primarily homo sapiens in admixture, the presence of a functioning neocortex is the best indicator of intelligence in the 0-95% range. Intelligence beyond that requires additional firepower and a strong corpus callosum (and other factors in interconnectedness).
Beyond the 95th percentile, modal intelligences become extremely important. See http://www.koanicsoul.com/blog/2012/07/08/high-iq-are-1-of-pop-half-neanderthal-outsiders-half-melonhead-winners/.
I described my theory of “modules” as CPU/GPU in this thread: http://drjamesthompson.blogspot.com/2013/08/gigerenzer-and-psychological-theorizing.html
Last, the size of a cortex is only related to its strength insofar as the size of an army is related to its strength. In both cases, a great deal of volume is dedicated to storage and support systems, rather than the processing (or “operations” units in the army). By the same token analogy, a peacetime (R-selected) army can be large without being functional, in spite of costs (trouble getting the head through the birth canal), but it is impossible to be truly strong without a large army. As always, intercommunication and harmony of purpose are key.