Wyoming currently has a population of about 600 thousand people, while California has a population of almost 40 million, yet each have two senators. Hence, it could be argued that a person in Wyoming holds about 68 times the power in the Senate than a person in California. Did the framers foresee such a large difference in representation? What was the largest relative difference in population between the original 13 states at the time that the Constitution was written? Is it much less than 68? Do any of their writings comment on the potential unfairness of this arrangement for states with radically different populations?
The non-proportional Senate and proportional House of Representatives is known as the Connecticut Compromise. It was the only way big and small states could agree in 1787 when the constitution was being written. It was very much intentional that the senate should give each state equal weight regardless of population.
In fact, equal representation in the senate was considered so important that it is the one aspect of the constitution that cannot be amended without the unanimous consent of the states (rather than the usual 75%). That is, the amendments article of the constitution provides that "no State, without its Consent, shall be deprived of its equal Suffrage in the Senate." (Of course, there are potential ways to work around this restriction, but that’s another question.)
The original apportionment (which was in place until the first census) of House seats was New Hampshire 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. So there was initially a ten-to-one ratio of House representation.
An important piece of context is also that the meaning of proportionality was not universally agreed upon. Democracy in 1787 was very far from universal suffrage. Only land-owning white men could vote in most states. Another issue that needed compromise was whether or not persons who could not vote count towards the state population for apportionment. The eventual compromise was that all free persons would count equally, while slaves were counted as three-fifths of a free person.
This proportion should not have surprised the founders.
The founders expected the House of Representatives to increase in size. (It has increased by roughly a factor of five.)
The founders would not have been surprised by one state having 1/10 of the total population. The founders also would not have been surprised by some state(s) only having 1 member of the House of Representatives each. Since those are the proportions of the California : Wyoming comparison, the founders should not have been surprised.
The Constitution also provides a means for addressing such an imbalance. If the legislature of a state and the Congress agree, a state can be divided into more than one state. For example: Virginia had the largest population during the American Revolution. It has since been divided into three full states plus parts of three others. And the Congressional resolution that made Texas a state contemplated that Texas might want to be broken into five states over time. As it is, half of New Mexico and parts of a few other states were originally part of Texas.
In effect, the Senate acts like an anti-trust law for the states. Just like anti-trust law prevents one company from buying up voting control of all of the other companies in its industry, the Senate prevents one (or a few) states from having voting control of the Congress. Just like anti-trust law encourages breaking up overly-large companies into competing companies, the Senate encourages breaking up overly-large states into competing states.
This incentive to avoid having overly large states is important. Some of the reasons that the Soviet Union fell can be traced back to Russia being a federation that had half of the population of the Soviet Union, and the alternative means that the Soviets used to limit Russia's power within the Soviet legislature.
Alexander Hamilton strongly objected to the equal representation of States in the Federalist #22:
The right of equal suffrage among the States is another exceptionable part of the Confederation. Every idea of proportion and every rule of fair representation conspire to condemn a principle, which gives to Rhode Island an equal weight in the scale of power with Massachusetts, or Connecticut, or New York; and to Deleware an equal voice in the national deliberations with Pennsylvania, or Virginia, or North Carolina. Its operation contradicts the fundamental maxim of republican government, which requires that the sense of the majority should prevail.
Sophistry may reply, that sovereigns are equal, and that a majority of the votes of the States will be a majority of confederated America. But this kind of logical legerdemain will never counteract the plain suggestions of justice and common-sense. It may happen that this majority of States is a small minority of the people of America...
There are some excellent answers here, but there is one point that I do not see represented, and that is that the framers did not want large population centers to have a potentially tyrannical hold over rural areas. They understood that inevitably the interests of large cities would differ from that of rural people's. The entire reason for the creation of a Constitutional Republic was intended to avoid the mistakes of a long history of failed 'pure democracies'. The tyranny of the majority was equally feared by the Founders as much as should be feared of an emperor or dictator. There's multiple quotes from several of the Founders on this topic.
Example "democracy is two wolves and a sheep voting on what's for dinner" - Ben Franklin
"The fundamental article of my political creed is that despotism, or unlimited sovereignty, or absolute power, is the same in a majority of a popular assembly, an aristocratical council, an oligarchical junto, and a single emperor. Equally arbitrary, cruel, bloody, and in every respect diabolical" - John Adams (1815) There are literally dozens more.
This is the same reason for the electoral college, which is to thin out the power of the high population areas.
A correct perspective of past times is, I think, the hardest thing for any student of history to acquire. The delegates gathered in Independence Hall represented 13 independent nation states newly liberated from being unique colonies with 150 years of individuality on one hand and as possessions of a mighty empire on the other. They were Virginians, Georgians, Carolinian's et al. Their initial effort, the Articles of Confederation, were steeped in fear of tyranny and sometimes merely called the League of Friendship. It had been designed to preserve the states sovereignty and restrict any central government. It succeeded in that end to a fault leaving 13 bickering little nations levying duties on their trade among themselves, printing their own money and unable to pay their war debts or otherwise raise money to note just a few problems.
Despite their problems the Philadelphia Convention had only been charged by the Confederation Congress to propose amendments to the Articles to strengthen it. They were the elite of their society. Well educated and certainly able to contemplate proportionality. They certainly understood the tyranny of the empire they had cast off and also feared the tyranny of any majority in terms of larger states.