Trying to understand how to factor in "fairness" here.

The fact is that irrespective of body content (ie fat or muscle), what counts for the economics of the airline is the total weight. Heavier the passengers and the luggage, more fuel is burnt, and the costs correspondingly go up.

I agree with you, however, that the 75 kg choice is pretty arbitrary. I wonder, though, if tickets online can be initially issued at price ex "weight-rate". Everyone would then stand with their luggage on the scales, and pay $k for every kilo (without any limit, per se). To me, that seems more "fair" without setting arbitrary limits that might carry discriminatory or racist overtones.