The word 大楼 is formed through a straightforward, conceptual combination of its two constituent characters. The first character, 大 (dà), means "big" or "large". The second character, 楼 (lóu), means "building" or "multi-storied house". By placing "big" directly before "building", the compound naturally and visually creates the meaning of a "large building", specifically evoking the image of a sizable, often modern, multi-story structure like an apartment block or office tower. It is a direct semantic sum of its parts.