Since fatness is colder because of the lack of sanguine vessels or of vitality, we may say that any expansion of body has a morbid feature.
The coldness shows itself morbid in the innermost part of the body. For instance, when we shiver because of coldness, the movements of the body come out from inside.
Meanwhile, the ailing thinness weakens the body until it becomes exposed to the outer influences. In fact, it becomes liable to the expansion of the surrounding things and it comes to them using its unhindered hotness of the sanguine vessels which are easily observable under the skin.
Thus, the leanness point to an outer morbidity of the world as such. Since the world cannot be entirely dead, the thinness keeps some vitality in spite of its morbidity.
And we may add that any accumulation of goods, since it is as if we would weight more, bears on a deep morbidity. The renouncement to them, though kills the social relations, still keeps a higher vitality.