Given that for 99% of our evolutionary history obesity was essentially impossible due to (a) an active rather than sedentary lifestyle, and (b) a constraint on available calories rather than a plethora of calorie-rich junk in every store, we need to be cautious about inferring too much from our ancestral history. We also need to remember that there's a significant difference between being fat (eg. overweight but not morbidly so) and being fat in the way modern humans have so recently become. A slight amount of fat would have signaled access to desirable resources, which is why plumpness was attractive in many third-world countries until quite recently. But obesity sends strong signals of unfitness across many domains (inability to cope with physical activity, high propensity for disease, unsuitability for pregnancy and birth, etc.) and so despite our presently PC attempts to pretend that obesity is merely "a valid body option" it may well be that our negative reaction to obese people is a much more deeply-rooted evolutionary adaptation than simply the consequences of pastoralism versus farming (which are in any case only relatively recent phenomena). We are highly adapted to find overt signals of sickness unattractive - hence also people's discomfort with skin diseases, maimed body-parts, etc.