Size is never a true indication of muscular power and efficiency. The smaller man usually makes up for the imbalance of power by his greater agility, flexibility, speed of foot, and nervous action. Bear this in mind once you go into action and grapple with an opponent: strive to keep him off balance, regardless of his size. So keep moving faster than he and pay absolutely no attention to his size, fierce facial contortions, or his vicious language. Your object is always to attack your opponent at his weakest points, which are mainly gravitational, throwing him off balance, and applying leverage principles so that his body, and the limbs of his body, are used to work toward his own defeat. "The bigger they are, the harder they fall."
In combatting a man with your bare hands, you must learn to use your head, knees, and feet as well as your hands. The "crowding"act gives you every opportunity to use these parts of your body, especially your elbows. Another simple method while crowding with your opponent is to step on his foot.
It has unexpected results. The one point to bear in mind when you are being attacked by a thug is the fact that the thug has but a one-track mind. He thinks but in one groove, which is bent on your destruction, barely considering what you can do, in which case you always have the psychological advantage on your side. With efficiency comes confidence and self-reliance.
Bruce Lee's handwritten essay from his pocket journal circa 1961.