thin, emaciated, lean, meager
瘦 centers on a lack of flesh or substance: being thin, emaciated, or lean, and by extension meager or scanty.
瘦 combines 疒 (sickness) with 叟, which likely contributes the sound and suggests a withered or aged appearance, fitting the idea of thinness from illness.
The sickness enclosure 疒 wraps around 叟, an old man. Picture an old man wasting away from illness, becoming thin and emaciated.
For ソウ, imagine a thin soul barely visible inside the sick body: soul -> ソウ, and the meager figure looks like a ghost.