"Tall man" is now accepted. Without 키, 큰 남자 is not exactly a tall man, but in the proper context 키 may be implied.
So it seems like ㄴ/은 is making adverbs until you realize these are all adjectives in English. What's going on?
In Korean, it's all just verbs. Tacking ㄴ/은 onto the stem of a verb makes it behave like an adjective instead. Like how slapping "-ing" onto the end of most English verbs can turn them into nouns.