Tony83,
Obviously I dont speak for Marcus, but I have also noticed the problem you mention.
Effectively, the population of any city will be will be about 4 times greater over 50-100 years than it would otherwise be with 2TPY. (All that compounding of pop growth). This is subject to a variety of game-imposed limitations, squalor effects, city improvements (especially farms), decrease in fertility approaching each city size stage etc, etc. If you halve the fertility rate, this creates issues with larger unit sizes and recruitment - especially for poor/barbarian factions. Also, halving the fertility, if fertility were the only factor, would result in less than half the growth. Given that fertility often contributes less than half to the total growth percentage, and that it seems you must use an integer, I have halved (FRD) the base farming levels/"fertility".
I am going to try reducing the fertility settings for each region as follows:
7-4
6=3
5=3
4=2
3=2
2=2
1=1
Each point is I think 0.5%.
I will also try reducing the trade value of grain to 1 (or less if possible) in the hope that this might limit the grain trade, which I understand may be a contributor if you have access to a sea trade route to a grain exporter.
Hopefully this will have some positive impact!
Otherwise I really like the new 4TPY mod with the increased building costs/times.
If any good comes of my test it I'll post it.
Regards