Basically it comes from this:

http://en.wikipedia.org/wiki/Ernst_Kretschmer

Kretschmer is also known for developing a classification system that can be seen as one of the earliest exponents of a constitutional (the total plan or philosophy on which something is constructed) approach. His classification system was based on three main body types: asthenic/leptosomic (thin, small, weak), athletic (muscular, large–boned), and pyknic (stocky, fat). (The athletic category was later combined into the category asthenic/leptosomic.) Each of these body types was associated with certain personality traits and, in a more extreme form, psychopathologies. Kretschmer believed that pyknic persons were friendly, interpersonally dependent, and gregarious. In a more extreme version of these traits, this would mean for example that the obese are predisposed toward manic-depressive illness. Thin types were associated with introversion and timidity. This was seen as a milder form of the negative symptoms exhibited by withdrawn schizophrenics. However, the idea of the association of body types with personality traits is no longer influential in personality theory.

In 1926 he became the director of the psychiatric clinic at Marburg University.