To be honest, I think you're wrong about it not taking conscious effort to stay skinny in modern society. Life has generally evolved under the threat of starvation and scarce resources. An animal might go days between meals: when they do make a kill they might gorge themselves because they don't know when their next meal might come. There were no supermarkets in cavemen days: early humans might have struggled to sometimes kill an animal and certainly wouldn't be able to do so every day. When they did make a kill, there was no refrigeration and they probably ate a bunch as quickly as possible to prevent it from being wasted. Basically, life has adapted for survival when resources are scarce, but hasn't adapted for modern society where resources are extremely plentiful. This survival mechanism saved lives throughout most of human history, but is a detriment in modern society and we haven't evolved out of that. On top of this, we're bombarded with sophisticated ads and foods are engineered with chemicals and sugar to make them as addictive as possible. So yeah, literally not eating a lot is all it takes. But this takes counteracting a lot of evolutionary as well as media programming. It's not exactly a small accomplishment to stay skinny in current year.