I don't know how you could be so wrong.

The south wanted slaves to count. That is the 3/5th designation. The EC was setup so plantation owners would be able to have as much power as their northern counterparts and the only way to do that was the EC and counting slaves.

I love the alternative history lessons though, just full of alternative facts.

P.S. There were no large/small states at that were vying for control, it was southern states with low(white male land owning) populations attempting to have their say, by allowing the counting of slaves.