If someone books a seat on a plane and ends up taking up a bit of another seat, United Airlines and a handful of other airlines may charge the passenger for both seats.

According to the airline, the decision was made for the comfort of the other passengers.

SURVEY: Charged Passengers Based On Size?

A self-proclaimed "fat advocate" said she's appalled.

"If a fat person buys a first-class ticket, the extra width of the chair is taken up by that little console, so basically the airlines build the chairs and airplane to say, 'We don't like fat people here,' and that's discrimination," she said.

So how is the airline going to judge who has to pay extra?

"What you are going to be charged for when you get there shouldn't be a surprise," attorney Alexander Anolik said.

Anolik wrote a book on travelers' rights. He said leaving the decision up to a flight attendant or gate agent is a bad idea.

"Subjectively, to say, 'I'm going to eyeball you. Turn around. You're going to pay for two chairs.' That's terrible," he said.

United officials said they received 700 complaints last year about uncomfortable rides next to large people. Records show 80 million people flew United last year.