The Relationship Between Size and Strength

When it comes to the relationship between a muscle's size and strength, there's more to the story than people think.  For example, this paper shows a lack of connection between the two.  For the arm muscle, there is only about a 23% correlation between size and strength.  Here's the actual graph:

The authors point out that the correlation is higher in the leg muscles, ranging from between 50 to 70 percent.  This may be because the leg muscles are primary weight-bearing muscles, and are used for walking, running, sports, etc., and receive different stimulation than the arm muscles.

So why would strength and size not be fully correlated?  I can't count the number times I've read that the cross-sectional area of a muscle is proportional to its strength, though this data (and other evidence) suggests this is not true.

For example, Bearpowered is a website run by speed coach Barry Ross.  His specialty is getting athletes to increase strength without increasing muscle mass.  In his "Hall of Fame", there are pictures of many people with slender physiques who are deadlifting pretty heavy weights.  Here's one picture of a person with a bodyweight of 144 pounds deadlifting 440 lbs:

I thought heavy weights equaled big muscle mass?  I guess not.  There is more to this story that I will explore in Part II.