Does someone have to be before you would verbally complain? Having been obese myself in the past, I know how hard it is. You already feel like everyone is staring at you and no one wants to sit next to you. Yea, in an ideal world you would buy two seats but we don't know everyone's situation to see if that is feasible or not.
I would much rather be quiet and "swallow my tongue" for a a few hours before complaining about the person next to me. I'm usually the "personal responsibility/libertarian" camp but in this case I think compassion is a better strategy.