Amending the Constitution is functionally impossible but adding states is very easy. The Senate is gravely tilted towards Republicans so at the present moment where Donald Trump has handed the Senate to Democrats, they must add as many states as possible. Maybe they can get to 25.
As GP said above -- a counterweight to to heavily populated areas. Which the more rural areas were at the time.
The more rural states (like Virginia at the time) had most of the population, which is why they preferred representation based on population.
The argument was one of population (whether current or expected), as writings by all involved in it will tell you explicitly.
Regardless, the US Senate was created with two senators from each state provided equal representation to each state in that house.
Which was and still is, a counterweight to heavily populated areas. Rural or urban really doesn't make a difference.
Edit: Clarified rural vs. urban importance WRT US Senate representation.