Population soft cap

What is the games soft cap (ie the point where the game will try to stabilize your population)? I have had a death rate roughly twice my birth rate for at least a decade now and my pop is over 90 million (I have not bought more then 3-5 M people in that time, if I even bought that many). Yet I still see 40k+/month expansion in my overall population, and that is consistent every month.

I am the Mindain Republica on White Giant.

I should add, according to the game, my net migration is only something like + 15 people/month, so that can't explain the population. I mean last year I should have lost 450,000 pop, but instead it went up by about that much. I am not complaining per say, I like that my pop isn't artificially declining, I just wish to understand the game better. (I played years ago when the soft cap was at 60M).