I can't find the quote I wanted, but it seems this was indeed a trend that my book (Becoming America by David Henkin) talks about. Example:
"Delegates from Virginia, where slave populations reproduced themselves naturally and slave- holders were no longer importing human beings from Africa or the West Indies, were willing to join New Englanders in criticizing the international slave trade, and Northerners from small states proved amenable to allying with small slaveholding states on that issue to thwart parts of the Virginia Plan."
What made Virginia different, for example, from the British West Indies or any other nation? How were slaves able to reproduce to such a degree, if they could?
There's been a lot of writing on this topic over the last 100 years, and I am hardly well-versed on the subject. However, there are few key points:
In most of the Southern states, the chief cash crop grown with slave labor was either cotton or tobacco. This is in contrast with the Carribean and Brazil, where black slaves were generally used in the growing of sugarcane. Sugarcane had a longer growing season, and the tropical climate meant that there was essentially no "rest season" -- the carribean slaves were worked very hard year round, and unlike cotton and tobacco, sugar was hard to make a profit on as a smallholder (fewer than 10 slave/workers), because of the large amount of post-harvest processing needed before it could be sold to middleman merchants. In contrast, tobacco and cotton were both less labor intensive crops, and in most of the South there is a distinct winter season in which agricultural activity was reduced. This gave slaves a respite to from hard labor, and gave the slaveowners less incentive to try to work the slaves to death to squeeze maximum profit out of them.