Have you guys been dynasty maxing?
In my Portugal game I am just before 1500 and I have around 300 living characters in my dynasty right now. One of the noble houses in Portugal also has like 200 living characters in it right now.
The noble marriage privlege removes the -10 legitmacy hit for marrying nobles. So if you just marray all your crown characters to nobles and the remaining nobles to each other they just explode in numbers.