You might be missing the joke in the OP.
In any case the meaning of "full" in "I'm full" is colloquial and not absolutist. It's more like "I'm satisfied" and not "I'm stuffed" (which also isn't used literally).
In any case the meaning of "full" in "I'm full" is colloquial and not absolutist. It's more like "I'm satisfied" and not "I'm stuffed" (which also isn't used literally).