Please see this thread in the strategy section.

http://forums.civfanatics.com/showthread.php?t=223910

Whipping is powerfull enogth how it is.
It is logical so as city grow efficiency of forced labor decrease. That how our civilisation develop in Real world, it is logical it will continue be so.
It is logical so as city grow slavery take less place and less efficient ties become good to work.
On size 20 slavery convert 1 food to 1 shield, making tundra forest as attractive tie to work as slavery. It is all good.

Maybe in BTS you'll be able to whip your empire into civil war.