sponge bath vs bathing

sponge bath and bathing both are nouns.

Word	Noun	Adjective	Verb	Adverb
sponge bath	Yes	No	No	No
bathing	Yes	No	No	No

As nouns, bathing is a hypernym of sponge bath; that is, bathing is a word with a broader meaning than sponge bath:

sponge bath: you wash your body with a sponge or washcloth instead of in a bathtub
bathing: the act of washing yourself (or another person)

Other hypernyms of sponge bath include washup.

sponge bath (noun)	bathing (noun)
you wash your body with a sponge or washcloth instead of in a bathtub	immersing the body in water or sunshine
	the act of washing yourself (or another person)