Every ten years, after each census is taken, the House of Representatives is reapportioned to make Congressional districts contain as mathematically equal a number of residents as possible. Originally, the House expanded in size to reflect population growth, but in 1929, the number of seats was fixed at 435. Thus reapportionment has required some states to gain seats and some to lose them. Each state must have at least one representative.
Donald Ritchie, Our Constitution