The reason the original colonies we able to unite together was making sure the smaller population colonies still had a voice and wouldn’t be drowned out by the larger population colonies. Still true today with smaller population states wanting a voice and not letting high population states steam roll over them and run everything.
And it had nothing to do with slave states having less population. All the states, save Pa, allowed slavery, and at the time of the Constitutional Convention slavery was waning. It had already been banned north of the Ohio by the NW Ordinance which was passed unanimously by the Confederation Congress. Slavery’s rebirth came from Whitney’s cotton gin and Slater’s textile mill. Both events post convention. As for population, until large scale immigration after 1830 the north and south were roughly equal and contrary to what has long been taught in schools, the Missouri Compromise’s requirement of an equal number of free and slave states came from a fear that slave state populations might outnumber the North. How else is Mass’s wiilingness to allow Maine to separate from it to become a state to keep the balance equal if the House, based on population was supposedly safely in Free state hands.