why not have a maximum total weight per passenger with excess charged on body weight as well as baggage ?
There are various bits of law which generally discourage discrimination against individuals. Ryanair have made noises about it on their website - but this is likely to be just a publicity exercise so as to get more mention by journalists (no such thing as bad publicity).
For those of a large physical size, airlines will usually provide a seatbelt extension for free. If however one's body size requires 2 or more adjacent seats, under certain circumstances (e.g. when the flight is fully booked) airlines sometimes require the passenger pays for *both* of those seats.