Fats love to shift the goalposts. To them, any human smaller than they are (no matter by how little a margin) is judged one of two ways:  If the person in question is a ham supporter or lover, they are considered to be "in slightly better shape" or "average" but always in minimal terms. This is usually accompanied by a self-assessment, again using minimizing terms, of themselves as "slightly above average" or cute phrases like "a bit on the heavier side." If the person in question is of the FPH variety, they are considered "skin and bones," "anorexic" or any other term that exaggerates the difference. In contrast to the former outcome, this comparison will cause either feigned self-indifference or a self-assessment of "I am healthy" or "just naturally bigger," but never any terms that indicate health issues.  So the underweight person in your scenario, for the sake of argument, would be treated with the latter option as the assessment is made on behalf of a person (you in this case) that is of the FPH variety, so the hypothetical underweight person must be considered an enemy and therefore deathly ill from lack of calories.