Honestly, wouldn't it make more sense for the rate to go up *faster* as more towns get fatter? Since you're slowly gaining more and more of a following for Dizzy's with each established restaurant
My logic was that the first jumps in the obesity rate are from the low-hanging fruit; the population that is most susceptible to becoming addicted and fattened by Dizzy's. As time goes on and the number of people who remain slim becomes a minority, those left are the more health-conscious, fitness-focussed types that are harder to corrupt. Especially when it becomes common knowledge how addictive and fattening Dizzy's is.
In a future update, I'll be expanding on this with the addition of an anti-obesity organisation fighting back against Dizzy's. You'll be able to lower the obesity rates, if you so desire, by working for this organisation, or you can work with Dizzy's to fatten it out of existence.