stuff vs fill up

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

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

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

Other hypernyms of stuff include fill, make full.

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

Other hypernyms of stuff include fill, make full.

stuff (noun)	fill up (noun)
miscellaneous unspecified objects
unspecified qualities required to do or be something
a critically important or characteristic component
senseless talk
information in some unspecified form
informal terms for personal possessions
the tangible substance that goes into the makeup of a physical object

stuff (verb)	fill up (verb)
fill with a stuffing while cooking	make full, also in a metaphorical sense
fill tightly with a material	become full
treat with grease, fill, and prepare for mounting	eat until one is sated
overeat or eat immodestly; make a pig of oneself	fill or stop up
cram into a cavity
obstruct
press or force