sesquipedalian vs polysyllabic

sesquipedalian is a noun but polysyllabic is not a noun.

Word	Noun	Adjective	Verb	Adverb
sesquipedalian	Yes	Yes	No	No
polysyllabic	No	Yes	No	No

As adjectives, sesquipedalian and polysyllabic are synonyms defined as:

sesquipedalian and polysyllabic: (of words) long and ponderous; having many syllables

sesquipedalian (noun)	polysyllabic (noun)
a very long word (a foot and a half long)

sesquipedalian (adjective)	polysyllabic (adjective)
(of words) long and ponderous; having many syllables	(of words) long and ponderous; having many syllables
given to the overuse of long words	having or characterized by words of more than three syllables