You have a larger proportion of outermost layer (skin, hair) and central nervous system than muscular or heavy persons. The brain and sense organs remain the same absolute size in the different body builds, but relative to the rest of the body, sensory mechanisms are of greater magnitude in thin persons. Constitutional psychologists categorize individuals with a greater proportion of outermost skin, like yourself, as ectomorphs, after the outermost layer of the body (ectoderm).