"소리가 크다" literally translates to "big sound", right?
알려 주셔서 감사드립니다
키가 크다 = big in terms of height (키) = tall
소리가 크다 = big in sound (소리) = loud
Is the -ㄴ/는다 form not applied for adjective verbs in writing? A native speaker once explained to me that the ㄴ/은(past) 는(present) ㄹ/을(future) doesn't apply to adjective verbs which only ever use ㄴ/은
In written Korean, 은/는 is also used as a Topic Marker. You will notice that it is attached to nouns rather than adjectives.