I am right in thinking that when you go up a population level the amount of food stored in that region resets to 0 again.
This can be a problem if that city suddenly comes under siege just after gaining a level.
I seem to remember in another game years ago with the same sort of population growth they had a mechanism where you only lost half the foodstock, you could therefore withstand a siege better.
If something similar was implemented here it would make sieges more realistic. A town wouldn't go from having tons of food stocks to zero suddenly.