Airlines don't charge you to cover any extra cost. They charge you just because they can. They will try at all costs to know how much are you willing to pay and

Airlines don't charge you to cover any extra cost. They charge you just because they can. They will try at all costs to know how much are you willing to pay and offer you that price minus one cent. If they think you are on a business trip they will try to charge you more, if they think you are on an emergency they will charge you more, if they think you are without options they will ask for your soul. It's true that fuel cost is a huge part of airline costs but your extra weight is going to change that very little. What really costs airlines money is operating half empty aircrafts, something that sometimes they are forced to do. Ticket prices are determined by the market, and airlines markets are very special.