thin out vs diminish

thin out and diminish both are verbs.

Word	Noun	Adjective	Verb	Adverb
thin out	No	No	Yes	No
diminish	No	No	Yes	No

As verbs, diminish is a hypernym of thin out; that is, diminish is a word with a broader meaning than thin out:

Other hypernyms of thin out include decrease, fall, lessen.

thin out (verb)	diminish (verb)
make sparse	decrease in size, extent, or range
become sparser	lessen the authority, dignity, or reputation of
lessen the strength or flavor of a solution or mixture