For functional strength, you can't really beat kettlebell and bodyweight workouts. Also, as others have mentioned, focus more on being dynamic against the stronger guys. Always fight the opponent in the aspect where they are weakest. Even if you can out-muscle them, why do it when there is an easier route. Also, get a basic understanding of anatomy. It doesn't matter how strong you are, the human body has functions in which it is stronger and more efficient and other functions where it doesn't function as well and is naturally weaker. Use the latter to your advantage. As Bobby said, Skeletal structure > muscles. Avoid having to burn your muscles out, and save those for the right opportunity.