It's worth keeping mind that for the majority of the history of the Senate, until the adoption of the Seventeenth Amendment in 1913, senators were not even elected by the voting population, but were elected by state legislatures.

Indeed, in the first couple of decades or so, state legislatures even tried sending voting instructions or recommendations to their senators (although as states didn't have any means of enforcing them, and Senators had longer terms than the legislators who elected them, there never was a means of enforcing this).

The adoption of a Senate in which each state had equal representation is of course a compromise - the famed "Connecticut Compromise" (named because it was spearheaded by Roger Sherman of Connecticut) that sought to reconcile Constitutional Convention delegates from large states that supported an upper chamber with proportional representation based on population (the Virginia plan, because of James Madison championing the proposal) and the New Jersey plan (named so because of William Patterson), supported by smaller states, that largely kept the Confederation structure as is, with representation solely on a state-equality basis, but with some enhanced powers. Interestingly, at the 1787 convention New York was a "small" state while Massachusetts was a "large" state, which I find interesting. How times change. Of course at least nine states needed to ratify the Constitution in order to activate the new governmental structure, and so there was a need to ultimately adopt a Constitutional system that was as broadly acceptable as possible.

So direct election of Senators is an amendment to an upper house that was itself a product of political compromise, and in many ways a holdover from the Confederation government. And not the only one: it hasn't been used since 1825, but if no Presidential candidate secures a majority in the Electoral College, the House of Representatives must elect a President, and the voting results are determined by state delegation, not by representative (ie California and Wyoming would each get a "vote"). As historian Jill Lepore said (specifically with regards to US elections), it's patches all the way down.

Now, about the fact that by 2050 70% of Senators are projected to be elected by 30% of the population. This has been making the internet rounds in the past month, and as far as I can tell it's as true as any projection of what 32 years from now will be like. But I'm actually not really sure if it's a huge change from how Senate vote distribution has always been. For example, I picked the 1940 US Census results, and at that time 70% of the Senators (68 out of 96) were elected by states that contained 37% of the population. This number includes most of the southern states, such as South Carolina, Alabama, and Mississippi, that held de facto one party elections in electorates that were highly restricted because of such Jim Crow laws as poll taxes and literacy tests. So you had situations like James Byrnes of South Carolina (a future governor and Secretary of State) being elected to the Senate in 1930 in a Democratic primary and primary runoff with around 250,000 votes being cast for all candidates, but in a general election with no opponent and with an astounding 16,211 votes being cast. And this in a state of some 1.8 million at the time!

Which is all a roundabout way of saying that the Senate has for much, if not all of its history, been rather unrepresentative of the US population as a whole, to say nothing of the potential voter population in a Senator's state. That this is something notable perhaps might be a new popular topic, but I will have defer to someone else more knowledgeable on that subject.

Of course if enough Americans get tired of equal representation in the Senate, that representation could be amended - in theory. Article V of the Constitution states that "no State, without its Consent, shall be deprived of its equal Suffrage in the Senate." In effect, this is the only part of the US Constitution that is unamendable, although some legal scholars have argued that this just means that changing Senate representation would require the unanimous consent of all states.

Well when the country was founded, the senate was not elected via popular vote, but chosen by state legislatures directly. So in that sense the senate controlled by a much much smaller percentage of the population.

In terms of represented population, the most populated state in 1790 was Virginia with ~450k (free) people, and the least populated was Delaware with ~50k (free) people, a ratio of 9:1. Compare that to 2010, where the most populated state (California) has 37 million and the least populated (Wyoming) has 550k, a ratio of 67:1. So while it was always skewed to benefit smaller states, that skew has grown massively over time.

The biggest reason is probably less related to what was best for the country overall and more related to each states interest. At the constitutional convention, each state had one vote. Enough small states banded together to force the issue and use "equal apportionment" for the senate (ie same number of senators per state) rather than proportional representation.

Some people were not exactly enthusiastic about this plan, but they wanted to keep small states in the union and believed this to be a necessary compromise. To whit, Federalist 62:

III. The equality of representation in the Senate is another point, which, being evidently the result of compromise between the opposite pretensions of the large and the small States, does not call for much discussion. If indeed it be right, that among a people thoroughly incorporated into one nation, every district ought to have a PROPORTIONAL share in the government, and that among independent and sovereign States, bound together by a simple league, the parties, however unequal in size, ought to have an EQUAL share in the common councils, it does not appear to be without some reason that in a compound republic, partaking both of the national and federal character, the government ought to be founded on a mixture of the principles of proportional and equal representation. But it is superfluous to try, by the standard of theory, a part of the Constitution which is allowed on all hands to be the result, not of theory, but "of a spirit of amity, and that mutual deference and concession which the peculiarity of our political situation rendered indispensable." A common government, with powers equal to its objects, is called for by the voice, and still more loudly by the political situation, of America. A government founded on principles more consonant to the wishes of the larger States, is not likely to be obtained from the smaller States. The only option, then, for the former, lies between the proposed government and a government still more objectionable. Under this alternative, the advice of prudence must be to embrace the lesser evil; and, instead of indulging a fruitless anticipation of the possible mischiefs which may ensue, to contemplate rather the advantageous consequences which may qualify the sacrifice.

(Emphasis mine. There are other sections of Federalist 62 which are more complimentary to the idea, probably because the point of this document was to promote the constitution as it had been agreed upon)

Sources: US Constitution Article I, Section 3: "The Senate of the United States shall be composed of two senators from each state, chosen by the legislature thereof, for six years; and each Senator shall have one vote."

Censuses of 1790 and 2010, see https://www.census.gov/library/publications/1793/dec/number-of-persons.html

Federalist 62 https://www.congress.gov/resources/display/content/The+Federalist+Papers#TheFederalistPapers-62