On airlines such as Southwest and United, people who can't fit into 17-inch seats with the armrests down and their seat belts fastened must buy a second seat or they don't fly. US Airways and American Airlines are likely to offer free second seats, but on a full flight they make extra-large passengers pay for them.Obesity is as often as not not a simple matter of gluttony or lack of will power. There is a ton of research going on that show there are dozens of factors, from viruses to air-conditioning to trace synthetic hormones. It seems pretty clear that in many, if not most cases of true obesity, too many Twinkies are not the cause.