The point is that energy storage in body fats is normal. Of course in hunter gatherers, there's a multitude of negative feedback mechanisms (e.g. you can't hunt when you're too fat), which presumably can interrupt the weight gain well before it reaches gargantuan proportions (i.e. "obese" BMI).

Ultimately, if you consume a little bit more calories than you spend, you gain weight little by little, and if that goes unchecked, over the years you will get severely overweight, obese, and so on.

edit: albeit maybe the food is to blame as well - it could be that the foods ended up engineered to be enjoyable at lower levels of hunger.