Indeed, there seems to be some confusion in this thread about how charging by weight works. It doesn't matter how tall or muscular you are, or what your BMI or body fat is. If you weigh more, you pay more.

It seems very hard to make it workable (weigh-ins at the airport). Plus, it's waiting for stories about people starving themselves to avoid extra charges and then collapsing on the plane.

If anything, I can see charges based on baggage weight becoming more granular. Ryanair already charge different amounts for checked baggage ($30 for a 15kg bag, $45 for a 20kg bag). Perhaps charging based on combined checked and carry-on is next, in 5kg increments.