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. Even if two individual are of the same strength, the heavier one will have the advantage. In general, heavier individuals usually have more natural strength as they carry their heavier weight in day to day activities.
While some smaller individuals are stereotyped as being faster, there are cases where the skinnier individual is weaker than normal while also being slow. This puts them at a huge disadvantage against anybody, let alone a much heavier person. These situations can present themselves too when the female is a lot heavier than the male when it comes to grappling.