fair
Explanation
The Joke
A person prays to God, asking "God, is life fair?" God responds from a glowing divine portal with a seemingly factual statement: "Men's height is correlated with both intelligence and penis size." When the person says "So..." God cuts them off with: "I'd explain but you're too short to understand."
The comic starts with the classic philosophical question about fairness and then answers it in the most brutally unfair way possible. God's response implies that life is decidedly not fair -- some people get advantages in multiple areas at once (height, intelligence, and genital endowment), while others get shortchanged across the board. The final insult, telling the questioner they are "too short to understand," is God simultaneously confirming the correlation and using it as an insult against the person asking.
The Humor
The humor comes from the triple-layered cruelty: first, God reveals that advantages compound rather than balance out (undermining any notion of cosmic fairness); second, God insults the questioner by implying they are short; and third, by extension, God is also implying the questioner is both unintelligent and poorly endowed. It is a perfect example of SMBC's irreverent take on theology -- God exists, but rather than being a source of comfort, God is basically a cosmic bully who rubs your face in life's unfairness.
References
The correlation between height and intelligence is a real, well-documented finding in population studies, though the effect size is modest. The correlation between height and penis size is more debatable and often exaggerated in popular culture.