Unfortunately some pax will use their body weight as an argument against paying excess luggage. I just smile and suggest if airlines start to go down that path, they could start to charge for obnoxious-ness, bad breath, body odour, or other personality traits and that would be very subjective..... and expensive if you annoy the CSO!!!!
If a CSO can, they will usually try to wangle seating to ensure that an extra large person has two seats to themselves, for everyone's comfort, but obviously that is not always possible. There is also the option for the pax to purchase a second seat, an option that many frequent travellers do do, again for their own comfort and to save embarrassment.
Cheers
Prado.