bolster vs fill up

bolster is a noun but fill up is not a noun.

Word	Noun	Adjective	Verb	Adverb
bolster	Yes	No	Yes	No
fill up	No	No	Yes	No

As verbs, fill up is a hypernym of bolster; that is, fill up is a word with a broader meaning than bolster:

Other hypernyms of bolster include fill, make full.

bolster (noun)	fill up (noun)
a pillow that is often put across a bed underneath the regular pillows

bolster (verb)	fill up (verb)
add padding to	make full, also in a metaphorical sense
support and strengthen	become full
prop up with a pillow or bolster	eat until one is sated
	fill or stop up