First of all, the size of a muscle does not necessarily represent its strength. The size of muscle mainly depends on muscle volume, while strength depends on the nerve control of muscle and how to use muscle fibers. The degree to which a person's nervous system controls muscles determines the level of strength that muscles can exert.
Second, the way of power output is also very important. If a person does not train or only trains certain muscle groups, their muscles may not be activated or coordinated. The activation and coordination of muscles can make muscles produce maximum strength output. Therefore, even if the muscles are large, if the coordination between muscle groups is not good enough, the power output will be affected.
Third, other factors will also affect strength, such as cardiopulmonary function, bone structure, flexibility and so on. These factors may limit a person's strength performance, even if their muscles are very strong. Therefore, strength is determined by many factors.