大きい (ookii) is an adjective that means large/big, whilst the kanji 大 alone is used more as a prefix and is mostly read as dai or tai (onyomi), in words such as 大学 (daigaku), 大変 (taihen) and 莫大 (bakudai) but still meaning large/big/great within the context of the word. That's my sloppy answer, but I hope it helps.