Size isn't everything

Chapter 11 endnote 31, from How Emotions are Made: The Secret Life of the Brain by Lisa Feldman Barrett.
Some context is:

Even if we examine many brains and find a statistically significant difference in insula size between people who are more or less aggressive, that doesn’t mean that a larger insula causes aggression, let alone murder.

Even if aggression were located in the insula, and lowered inhibitions were located in the anterior cingulate cortex (they aren't), just how much bigger or smaller than the norm would those brain regions have to be before we would consider them as a causal explanation? Most people with different-than-normal brain regions don’t commit murder.