plump vs fatten up

plump is a noun but fatten up is not a noun.

Word	Noun	Adjective	Verb	Adverb
plump	Yes	Yes	Yes	Yes
fatten up	No	No	Yes	No

As verbs, plump and fatten up are synonyms defined as:

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

plump (noun)	fatten up (noun)
the sound of a sudden heavy fall

plump (adjective)	fatten up (adjective)
sufficiently fat so as to have a pleasing fullness of figure

plump (verb)	fatten up (verb)
give support (to) or make a choice (of) one out of a group or number	make fat or plump
make fat or plump
set (something or oneself) down with or as if with a noise
drop sharply

plump (adverb)	fatten up (adverb)
straight down especially heavily or abruptly