Airlines sell seats. Before I spout, I am 12 stone and can fit into any seat offered me. I do not know anyone close to me who is grossly overweight.
I feel that large people should not be discriminated against, many cannot help being large and, seats to accommodate them should be made available, don't ask me how, it's not my job.
If it is deemed necessary to charge them more, then that passenger has a choice.
Safety should take precedence over profit. imho.