First of all, I want to say that this has nothing to do with your son, Mike Furches. This is just something that I have noticed with football. It helps to be fast but just because you are the fastest does not mean that you are the best runner on the team. You also have to have the right technique to run the ball like cutting up the field, making good decisions and being able to "see" the field.
As far as size, my husband was an excellent defensive tackle on a winning state football team. He is only 5'6" tall. Again, his technique on the field is what helped him succeed.