Also, from Wikipedia on Baker v. Carr:
Having declared reapportionment issues justiciable in Baker, the court laid out a new test for evaluating such claims in Reynolds v. Sims, 377 U.S. 533 (1964). In that case, the Court formulated the famous "one-man, one-vote" standard for legislative districting, holding that each individual had to be weighted equally in legislative apportionment. The Court decided that in states with bi-cameral legislatures both houses had to be apportioned on this standard, voiding the provision of the Arizona constitution which had provided for two state senators from each county, the California constitution providing for one senator from each county, and similar provisions elsewhere.
What was their rationale? Did they just invent it out of thin air? If anything other than "one man one vote" would violate Fourteenth-Amendment "equal protection", why would later amendments be needed to give suffrage to blacks and women? It is totally implausible to suggest that the Founding Fathers were opposed to the idea of representation not based on population, since that concept is enshrined in the Constitution itself.