I was flying from Seattle to Spokane. I arrived at the airport early and put myself on the standby list for an earlier flight. I was given a seat on the earlier flight. Shortly after the flight began to board I was called to the desk. They took the seat back from me in order to accommodate a passenger who was too large to occupy just one seat. I realize that I don't have much standing as a standby passenger but I thought that passengers who were too large for one seat were required to purchase 2 seats.