There is a lot more to strength than muscle size. Muscle size is a very very small component of strength in compound movements. In isolation movements where you are working/testing a single muscle, then size is a bit more important but even then it is not paramount - it is activation. You want to activate as many muscle fibers in sync with one another to produce maximum force output. In compound movements, this is even more important as several muscles must synchronize together to output a force. This is a bit of a simplification, but it does the job in illustrating there there is much more to strength than muscle size. In fact, most studies (and practical applications) are highly focused on the fact that most, if not all, strength gains within the first few months to years of training are from neurological adaptations and not correlated as well with muscle mass increases.
