It sounds like a no-brainer that if you eat more you'll gain weight. Given that 1 kg of fat contains 8000 kcal of energy, you can estimate how much weight you'll gain if you eat more. But there are reasons to be sceptical here. There seems to be experimental proof against this simple model that assumes that the metabolic rate does not adjust itself to the energy take, see e.g. here.
Then there is a theoretical argument against the simple model. Animals living in the wild have to cope with whatever is avaialable to eat. If they find temselves in a situation where they need to expend a bit more energy to get to their food and what they then get isa bit less, they would eventually starve to death if their metabolic rate would not compensate for that. So, if you eat one sandwich of 100 kcal a day less and keep all the other factors the same, you should lose 1 kg every 80 days, which means more than 45 kg weight loss in ten years. But this obviously not going to happen.
It also cannot be the case that people will feel more hungry if they eat a bit less, because some people like me stick to a diet where they measure everything they eat and their weights typically does not fluctuate wildly on a time scale of years. Nevertheless, the calories in mines calories out = weight gain model is widely used, but it seems to me without any solid scientific basis.
My personal observations suggests that people who are obese don't eat all that much, rather they tend to eat unhealthy foods and they are physically unfit. In contrast, I eat a lot, about 4000 Kcal/day, but I'm physically very fit and I only weigh 57 kg. I eat a lot more than everyone I know, but I stick to eating only healthy foods.
I would be most interested in experiments where healthy, physically very fit people were given a lot more to eat, but only healthy foods. E.g. larger portions of brown rice at dinner, more whole grain bread at lunch, more vegetables fried in olive oil etc. while their exercise routine is monitored before and after the intervention so that the energy expenditure change due to voluntary factors is taken into account. A steady weight gain according to 8000 Kcal energy surplus = 1 kg in such an experiment would prove the simple model.