2016 Postmortem

At the time the Constitution was passed, most states were slave states. MA freed all states in 1783 by state SC decision, the first state to do so, although VT had tried (it was unenforced) earlier. The process had only begun in PA, NH, CT & RI by 1787 (new Constitution). It was not until the 1800s that most northern states succeeded in actually freeing all their native slave population.

And in any case, the electoral college prefers small population states over large population states.

The electoral college does not allocate votes strictly by population, but rather by Congressional representation (number of a state's Representatives in the House plus 2 Senators). This is not historically due to slave/non-slave state issues, but to the smaller states knowing that they would essentially be the garbage dump for the others without this arrangement.

The slave state thing came in when the slave states did not allow slaves to vote, but demanded that the representatives be allocated including the slave population. The compromise was the 3/5ths deal, which of course vanished into the trash heap of history as a result of the Civil War.

Your beef is not the electoral college, but the representation of the Senate, which provides smaller states more weight in the election than you would think fair. What citizens of RI or WI might think about the matter might surprise you.

Although the original government of the States (Confederation) did not work well, the barrier to creating a stronger Federal union was the concern of the small states that they would essentially have no say in government of that federal state.

One look at the First Congress of the Unites States (1789) shows that the electoral college in no way preferred states that would turn out to be slave states - instead it reduced their influence:
https://en.wikipedia.org/wiki/1st_United_States_Congress#House_of_Representatives_3

The Carolinas and Virginia had 20 seats in the House, out of 64. It was in the SENATE that the smaller northern states had power, because RI had two seats, just like VA.

The US Constitution establishes a split representational scheme varying from degrees of represention by population, from most to least:
House
Presidency
Senate.

It explicitly establishes a president selected by states rather than popular vote. At the time, this was more about power-sharing than slavery. It still is.