Marriage to non-Jews might be an explanation. They may have progeny, but have not stayed with that religion. Or, like most folks these days (aside from Muslims), they just have one or two. Our culture doesn’t approve of lots of kids.
From approximately 10 Million Jews coming out of the Holocaust, to 16 Million in 2022? Most of them live in the US. So Israel does not have the numbers to defeat any modern day invasion. The population of NYC is close to that number of Jews. Keeping in mind that if you marry a Jew, you become one as do your kids. If of course, you have a traditional marriage and one partner.
If it were 10 million Italians, Irish, Hispanics, Catholics, or blm people in 1940. There would be 500M in 83 years. It just seems like a strange thing.