slim vs slim down

slim is an adjective but slim down is not an adjective.

Word	Noun	Adjective	Verb	Adverb
slim	No	Yes	Yes	No
slim down	No	No	Yes	No

As verbs, slim and slim down are synonyms defined as:

Other synonyms of slim include melt off, reduce, slenderize, thin.

slim (adjective)	slim down (adjective)
small in quantity
being of delicate or slender build

slim (verb)	slim down (verb)
take off weight	take off weight