From 1789 to 1909, the house of representatives grew in size because membership was based on the idea of equal representation. the framers of the constitution planned it that way. more western states had been admitted to the union. the population of the united states grew during those years.

The correct answer is "the population of the United States grew during those years".

Let's take the timeline of the House of Representatives number of members.

In 1789 had only 65 members, the original House.After the 1790 Census, the House was expanded to 105 members.The number of seats increased to 142 members after the 1800 Census.The number of current seats (435) has not been increased in more than a century, even though every 10 years the census shows the population of the United States grows, because of the Permanent Apportionment Act of 1929, that is the outcome of a controversy between rural and urban areas after the 1920 Census.