Last time I flew I tried to buy the seat next to me, full-fare, because I'm "big boned". The airline refused to sell it to me.
Go whine at them.
Not only do they squeeze the neighbors out of their own seats but they resent that the neighbor’s body parts are making the fat person uncomfortable, you know like your nose is denting his arm and your glasses are making an imprint on his skin. Many Pachydermal citizens have bad attitudes on airplanes to begin with because they think the plane should be made with all 30 inch seats because one or two extra large seats would be insulting and charge not one cent more for a ticket. I have heard these arguments and watched people made very miserable by being seated between two of them. I have not had that misfortune myself.