thin vs slim down

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

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

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

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

thin (adjective)	slim down (adjective)
of relatively small extent from one surface to the opposite or in cross section
relatively thin in consistency or low in density; not viscous
lacking excess flesh
(of sound) lacking resonance or volume
not dense
very narrow
lacking spirit or sincere effort
lacking substance or significance

thin (verb)	slim down (verb)
take off weight	take off weight
lose thickness; become thin or thinner
make thin or thinner
lessen the strength or flavor of a solution or mixture

thin (adverb)	slim down (adverb)
without viscosity