Large bodies are warmer, all else equal, if largeness is due to fat or muscle. But large bones probably don’t make a difference. Regardless of being large, your skin will feel about as cold and your risk of frostbite is about the same. And fat usually accumulates when you do less exertion. Since exercise tires you out sooner, you tend to become more sedentary, and you may be colder.
The Main Stuff
The fat and muscle help insulate vital organs against hypothermia. Large muscles warm more blood. Warm blood going through the extra fat or muscle stays warmer longer. But you also have more skin, so while the blood arriving at your skin is warmer due to your having more muscle or fat, at your skin it’ll be exposed to outside cold longer.
At any rate, being substantially overweight or obese puts your health at risk for other reasons, so I don’t recommend staying there, if it’s a choice and if health is your only consideration.
On the other hand, being just a little overweight may help your brain’s health.
Bariatric surgery, designed to help people who are overweight or obese lose weight and keep it off, may affect your ability to cope with hypothermia.
If the weight-loss surgery lowers your dietary intake, you still need to consume enough to stay warm. Eat and drink highly efficient foods, with enough nutrients. Whether you need carbs or calories, get what you need. Steer clear of junk food.
If the surgery makes physical exertion easier, because you can carry your weight more easily, the exertion will help you to stay warm, too.