Are there really any medical conditions out there that cause people to become fat?

Like, take an average woman, 5’8” who needs to consume 1500 calories a day to maintain her weight (any more than 1500 she gains, any less than that she loses weight.) If she stuck to that calorie intake, is there really a medical condition that would actually cause her to gain weight instead of maintain it? I don’t see how that could be. I just don’t see how a woman eating 1500 calories a day could end up weighing 300 pounds. Sure, metabolism plays a part, but in that instance I’d think that simply cutting back to say, 1200 calories would counter balance that.

And if there is a medical condition, how come only the populations in the well fed, developed nations are prone to it? You don’t see obese people any where in the starving populations of, for example, Somalia.