I am playing the IGC v1.9 on version 1.8BEN. Playing with Russia, I wrote down each month for one year the population of Moscow, it seems the growth is the growth of people per year per thousand. So if you have a growth of "12%" this would mean your population would multiply with a factor 1.012 . So a city of 100000 would be 101200 next year.
Hope this helps
Hope this helps