There are 3 different types of frame -
big frame - runners like Solinsky, Mottram, Purrier, True, etc.
average frame - most runners
small frame - KoKo, Jager, Kiprop, Geb, etc.
There is a test, where you use your left hands index finger and thumb to wrap it around the right wrist - if it overlaps, it indicates small frame, if it just touches, average, if the fingers don't even connect very big frame.
Why is this important? Because the ideal weight is different for each athlete based on his frame. A small framed runner needs to be 5-10 lbs below the ideal weight of an average frame runner, and a big frame runner needs to be 5-10 lbs above ideal weight.