butterball vs fatso

butterball and fatso both are nouns.

Word	Noun	Adjective	Verb	Adverb
butterball	Yes	No	No	No
fatso	Yes	No	No	No

As nouns, butterball and fatso are synonyms defined as:

Other synonyms of butterball include fat person, fatty, roly-poly.

butterball (noun)	fatso (noun)
small North American diving duck; males have bushy head plumage	a rotund individual
a rotund individual