My BIL is literally twice my weight,if not more; and has several inches of height on me.

Severity can be promoted by need. For example, his drunk ass wanted to get off the couch to threaten his sister (my wife). I introduced him to the floor, rapidly.

Speed and skill can overcome size. That being said, I’ve always been on the smaller side of stature and weight, and a hit from someone twice your size hurts when they connect. Period.