Big people comes in all shapes and sizes. You will have the sedentary obese individuals, or just the naturally big individuals. While they store more fat than regular people, they also tend to hold more muscle and denser bones. Regardless, there is no doubt that heavy people are usually stronger than naturally skinny individuals.
There is a reason for this, and it’s a simple one. The bigger you are, moving your body becomes a work out. Eventually, your body will become accustomed to it, unless you are one of those immobile obese individuals. When a lighter mass pushes on a larger mass, there is significant recoil. This is one of the biggest advantages of a weight advantage in anything involving striking and grappling.