money vs big money

money and big money both are nouns.

Word	Noun	Adjective	Verb	Adverb
money	Yes	No	No	No
big money	Yes	No	No	No

As nouns, big money is a hyponym of money; that is, big money is a word with a more specific, narrower meaning than money:

Other hyponyms of money include big bucks, bundle, megabucks, pile.

money (noun)	big money (noun)
the most common medium of exchange; functions as legal tender	a large sum of money (especially as pay or profit)
the official currency issued by a government or national bank
wealth reckoned in terms of money