Recently two 102 year olds passed away, R' Elyashiv and Benzion Netanyahu (the father of the Prime Minister). Although they lived to the same age the number of descendents that they have is radically different.
Benzion Netanyahu has 12 descendents, 2 of them great grandchildren. This makes sense as he had 3 children, 1 of the three (Yoni) had no kids and the other 2 (Iddo and Bibi) had 2 and 3 kids respectively.
R' Elyashiv on the other hand is estimated to have at least 1300 descendents. The math is actually quite simple. If you take an average family size of 6 (which may be small, R' Elyashiv himself had 12 children of which 10 survived to adulthood) and multiply it 3 additional generations, you get 6 * 6 * 6 * 6 = 1296. The actual number might be higher as we know R' Elyashiv had great great great grandchildren (a fifth generation)
These numbers explain why the Chilonim are worried, sooner (rather then later) Charedim will be a majority in Israel.