City Planning project claims to double food consumption, does not actually do that consistently.
Build r20334 (dev)
Seems to use from no extra food to the listed twice.
Farin I -> 2,526,071 humans and 2,008,868 Haduir, 5 food use, no change when doing city planning.
We Made It I -> 5,448,462 humans, food use goes 5 to 7.
Kobold I -> 2,969,834 humans, food use goes from 3 to 4.
I can't tell the pattern here. Although, I have not yet found a fertile world that cost less than the listed 2x multiple, so it might have something to do with low fertility worlds.