fill out vs fatten out

fill out and fatten out both are verbs.

Word	Noun	Adjective	Verb	Adverb
fill out	No	No	Yes	No
fatten out	No	No	Yes	No

As verbs, fill out and fatten out are synonyms defined as:

Other synonyms of fill out include fat, fatten, fatten up, flesh out, plump, plump out.

fill out (verb)	fatten out (verb)
become round, plump, or shapely	make fat or plump
make bigger or better or more complete
write all the required information onto a form
make fat or plump
line or stuff with soft material
supplement what is thought to be deficient