There are many variables which contribute to a muscle's ability
to produce force.
Architecture:
The Orientation of muscle fibers influence force production
capabilities per square unit. For example, the diagonal orientation
of the gastrocnemius (calve muscle) gives it greater ability to
contract versus the linear orientation in the biceps brachii.
The way in which a muscle crosses one or more joints can give it
a mechanical advantage relative to other muscles. The muscular
system is essentially a lever system. Compare the moment arms
between two muscles and you can see which one has the advantage
Physiology
Muscles are comprised of several motor units, some have the
ability to contract faster and with more force than others. This is
influenced by training and genetics. Muscles with more "Fast
twitch" muscle can produce more force than "slow twitch" muscles.
Think Olympic weightlifters versus endurance athletes.
A larger muscle generally has more contractile elements than a
smaller one. This is also due to genetics and training.
"Myofibrillar Hypertrophy" seen in barbell sports makes a stronger
muscle than "Sarcoplasmic Hypertrophy" seen in bodybuilding. The
former increases the actin and myosin within the muscle, the former
increases the amount of water in the sarcoplasm.
It's somewhat erroneous to think of muscles as individual units.
This is great for learning basics of anatomy, but in practice,
muscles always work in concert with each other, interdigitate with
each other and only really become separate under the scalpel.
