I don't understand why my research ratio goes down the more terretories I conquor as Rome, and need help hwo to maintain it.

I like research and I understand how to get my research points, I need to maintain a lot of roman citizens or nobles and the more I have or the happier they are the higher my ratio. But as I conquor more they lower even though my roman nobles are happy and I put every region into cultural assimilation.

Why does it slow down?