The way the U.S. House of Representatives is to be constituted is specified in Article 1, Section 2 of the U.S. Constitution.

U.S. Constitution (Article 1, Section 2)

The House of Representatives shall be composed of members chosen every second year by the people of the several states... No person shall be a representative who shall not have attained to the age of twenty-five years, and been seven years a citizen of the United States, and who shall not, when elected, be an inhabitant of that state in which he shall be chosen.

Representatives and direct taxes shall be apportioned among the several states which may be included within this Union, according to their respective numbers... The actual enumeration shall be made within three years after the first meeting of the Congress of the United States, and within every subsequent term of ten years in such manner as they shall by law direct. The number of representatives shall not exceed one for every thirty thousand, but each state shall have at least one representative; and until such enumeration shall be made, the state of New Hampshire shall be entitled to choose three, Massachusetts eight, Rhode Island and Providence Plantations one, Connecticut five, New-York six, New- Jersey four, Pennsylvania eight, Delaware one, Maryland six, Virginia ten, North-Carolina five, South-Carolina five, and Georgia three.

Over the years, there has been (and continues to be) many battles over apportionment of the U.S. House of Representatives.

Apportionment Timeline