From 1789 to 1909, the House of Representatives grew in size because the population of the United States grew during those years, too.
Congress often increased the number of seats in the House of Representatives based on population growth, and 435 is the most recent count.
The first House of Representatives in 1789 had only 65 members.
After 1790 it was expanded to 105 members.
After 1800 there were 142 members.
Nowadays, there are 435 members.