1. Im a little confused about your example... why exactly is Timmy copying Jimmy? Memetic evolution is a selective process, so for Timmy to want to copy Jimmy there has to be an incentive. I will proceed assuming you mean that societal pressure provides an incentive for this strategy..i.e. people make fun of Jimmy for being fat.
2. You are assuming that lower food consumption is the culturally favorable strategy(i.e. being 'thin' is a cultural virtue at the moment). I dispute this, at least for males. The modern ideal for males is more of a large 'athletic' physique than a small 'thin' one or large 'obese' one. So while there is disincentive to eat tasty, but unhealthy food(bad carbs+fat), there is great incentive to eat a lot of healthy food. This 'healthy' food(protein+good carbs) is more expensive from a resource pov, and will if anything further tax food supply.
