Some people have misconceptions about what it really means to be strong. You don't need big muscles to be strong. That is, and always will be, a false claim. The ability to generate tension in your muscles isn't related to muscle size, but rather, the efficiency of your nervous system.
The central nervous system (which you can think of as your control center or operations manager) dictates how hard you can tense your muscles.
The more you practice moving against resistance, the more tension your central nervous system allows. The body naturally seeks efficiency and will adapt to physiological demands placed upon it (which is known as the law of adaptation to imposed demands).
As long as you continue to move against resistance, your central nervous system will aim to increase the efficiency of that movement. It does so in large part by allowing for more tension in the muscles.
Take for example, the gymnast, who may be, pound for pound, the strongest athlete in the world. The classic gymnastic physique isn't overly muscular, certainly not bulky, yet a gymnast can display nearly superhuman levels of strength.
Why? Well, the gymnast has a finely tuned nervous system, which means the gymnast's movement is efficient. The gymnast has learned the skill of strength, so to speak. And that's all strength really is, a skill.