For what it's worth.....
Airlines can impose any baggage weight limitation they wish, and charge as they wish.
Passenger weights are averaged. The following values are used in the EU (JAA, Joint Aviation Authority) for aircraft with more than thirty seats:
all flights except holiday charter - adult 84 kg
holiday charters - adult 76 kg, children 35 kg
Discrimination with reference to body weight is not permitted. Hovever, if a passenger is so large that two seats are required, then two seats must be paid for.