Most airlines in the US have an official policy allowing them to require passengers who cannot sit in a single seat with the armrests lowered and seatbelt faste

Most airlines in the US have an official policy allowing them to require passengers who cannot sit in a single seat with the armrests lowered and seatbelt fastened to pay for an additional seat. They usually try to employ alternatives, but on full flights, people do get bumped if they don't fit in one seat.