I don't mind it so much on public transportation. The trip is usually pretty short, so even if I have to stand, it is no big deal.
On an airplane... that's a different story - there is no standing (and my flights are usually just within the US - but a 5 hour flight from California to New York is a long time) so when I can't use the space I paid for because someone's extra large, that is irritating.
It should be the responsibility of the oversized customer (they know who they are, especially if they have previously flown) to alert the airline/travel agent/online booking web-site when they purchase their ticket. If it means they get two seats, well, it is what it is - whether the airline chooses to charge the one person for the two-seats together is up to the airline.