fatten up vs fill out

fatten up and fill out both are verbs.

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

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

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

fatten up (verb)	fill out (verb)
make fat or plump	become round, plump, or shapely
	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