Humor is subjective, and countless factors typically contribute to what a person finds funny. In this particular sentence, "When she said she was too big to sit on your face but you didn't listen," the humor lies primarily within the realm of irony, surprise, and situational comedy.
Irony, as a form of humor, often involves a reversal of expectations or the contrast between appearance and reality. In this sentence, the irony lies in the protagonist's failure to heed the woman's warning. It's commonly assumed that people would typically listen to a caution that directly impacts their physical well-being, but the protagonist does not, leading to an unexpected outcome, therein lies the irony.
The humor also falls under surprise theory. Surprise theory suggests that something becomes funny when we anticipate one outcome but get another. In the above phrase, one would think that the man would heed the woman's warning about her size. Still, the surprise comes when he doesn't, resulting in presumably comical consequences.
Moreover, it offers a twist on the traditional “didn’t listen or heed to warnings” trope. People are usually warned of serious dangers, but the nature of this particular issue is on the absurd side. Comedy often comes from exaggeration, and this involves the exaggeration of personal weight as a potentially dangerous or problematic factor.
Another element of humor could be derived from a touch of schadenfreude, pleasure derived from another person's misfortune, seen from an external perspective. The amusement springs from the image of the man suffering the consequence of his refusal to listen to the woman’s warning. Yet, the situation is so far-fetched and unusual that it strays into humorous territory.
Finally, the humor in this sentence may also derive from its frank sexual implication prompting an awkward situation. The sentence describes an inherently intimate scenario, but flips it into a ludicrous circumstance by involving the largely unrelated issue of weight. This detachment from reality, combined with a touch of absurdity, may generate laughter. WHEN SHE SAID SHE WAS TOO BIG TO SIT ON YOUR FACE BUT YOU DIDN T LISTEN