Looking at the statistics from my own tree, I can tell you that how prolific a family is varies wildly.
My Stanley (Standley) side was extremely prolific. Gt-gt-gt grandpa Standley, born in the late 1790s, had 9 children. I have tracked down about 85% of his gt-gt grandchildren (my father's generation), and estimate that there are about 600 of them. I've estimated that my generation must have about 1600, and there are grandparents and probably even great-grandparents among us. So there are undoubtedly THOUSANDS of living descendants from just these TWO gt-gt-gt grandparents, and that Mrs. Standley was one of about 10 children, herself. Her family was generally just as prolific, so there easily could be 10,000 descendants of just my Heckathorn 4gt grandparents out there - 5th cousins, give or take.
The distribution of those descendants is wildly uneven. One of the Standleys' children had a small family, and there are ZERO known descendants. At the other extreme, the descendants of children #2 and #3, who had especially large families, account for about half of the overall descendant count. Basically, they had twice the average number of descendants of their siblings. Their generation is from the same time period as your gt-gt-gt-gt grandparents.
At the other extreme, my gt-gt-gt grandfather Cronin was born in 1800, and had at least 9 children, too. But 2 of those 9 children died before they got to be 5 years old. 2 others died in their 20s, without having married.
Of the remaining 5 of the Cronin children, one daughter had 8 children. They all lived to adulthood, but only 3 ever got married, only 2 of those 3 had any children. Despite having 8 children, there were only TWO grandchildren.
Another of the 5 had 4 children, but only 2 married; only 4 grandchildren there. There was also one who had only 2 children, one of whom had none of her own - only 2 grandchildren there. Another had 4 children, but only 2 grandchildren.
That leaves my own gt-gt grandfather Cronin, who had 6 children. Two died as children, two simply disappeared as young adults (foul play suspected in at least one case), and one was a daughter unable to have children. That leaves ONLY gt-grandpa Cronin, who had a bunch of daughters, 9 grandchildren, and dozens of gt-grandchildren (including me).
I get the impression that the bulk of the descendants of the original Cronin gt-gt-gt grandparents are my more immediate relatives, out to my 2nd cousins. Maybe a hundred or two living descendants - nobody more distantly-related than a 2nd cousin has been identified in my matches.
So, despite both being born around 1800, and having 9 kids, one has thousands of living descendants, while the other has maybe a couple of hundred. As you can see, it's not even just about having a bunch of kids - it's about whether they live to adulthood (about 1 in 6 didn't, back then), actually get married (well, back THEN!), and then actually have a bunch of kids of their own. Many hurdles!