There is a hard cap on population at 300M.
The country will generally stop growing of its own accord somewhere between 40M and 60M depending on a bunch of factors.
After the point where it stops growing of its own accord, I believe it will very slowly lose population. The birthrates will be used by the system to attempt to keep population stable at any level though so there shouldn't be any fundamental difference between a 74M population country and a 300M population country.
However, I haven't bothered to attempt to reach such high levels so this could be inaccurate and others would probably know more.