swell vs belly out

swell is a noun but belly out is not a noun.

Word	Noun	Adjective	Verb	Adverb
swell	Yes	Yes	Yes	No
belly out	No	No	Yes	No

As verbs, belly out is a hyponym of swell; that is, belly out is a word with a more specific, narrower meaning than swell:

Other hyponyms of swell include distend, belly, blow up, puff, puff out, puff up, bloat, blister, vesicate.

swell (noun)	belly out (noun)
a crescendo followed by a decrescendo
the undulating movement of the surface of the open sea
a rounded elevation (especially one on an ocean floor)
a man who is much concerned with his dress and appearance

swell (adjective)	belly out (adjective)
excellent

swell (verb)	belly out (verb)
expand abnormally	swell out or bulge out
cause to become swollen
increase in size, magnitude, number, or intensity
come up, as of a liquid
become filled with pride, arrogance, or anger
come up (as of feelings and thoughts, or other ephemeral things)