petty vs junior-grade

petty is a noun but junior-grade is not a noun.

Word	Noun	Adjective	Verb	Adverb
petty	Yes	Yes	No	No
junior-grade	No	Yes	No	No

As adjectives, petty and junior-grade are synonyms defined as:

Other synonyms of petty include lower-ranking, lowly, secondary, subaltern.

petty (noun)	junior-grade (noun)
larceny of property having a value less than some amount (the amount varies by locale)

petty (adjective)	junior-grade (adjective)
inferior in rank or status	inferior in rank or status
contemptibly narrow in outlook
(informal) small and of little importance