Why was the Russian Empire predicted to have insane population growth if it didn't collapse?

This would not have brought down birth rates dramatically to the levels of post-1970 post-industrial societies,
I don't understand where the problem. Any demographic forecast for Russia of the 20th century considers down the decline of fertility and mortality associated with the demographic transition.

Ireland had high birth rate ,but a declining population due to emigration.
Although emigration from Russia will be 2–3 times larger than OTL, it will not particularly affect demographic indicators. Most likely, the immigration of the non-European population in the late 20th - early 21st centuries will be a more significant political and demographic factor
 
Top