니만큼

'Given that it is X…'

-니만큼 gives a reason and implies the response should match its weight: 오늘은 좋은 날이니만큼 'since today is a special day (we should act accordingly).' It very often doubles the noun for emphasis — 주제가 주제이니만큼 'given what the topic is', 날이 날이니만큼 'given what day it is' — which leaves the obvious part unsaid. It sounds a little more composed than plain 'because'.

Register: slightly formal, spoken